Narrative:

We were taxiing out as flight X going. We were cleared to follow a regional jet to the runway. I thought the ground controller was a little garbled when our call sign was used but the first officer read back the clearance with no problem from ATC. Since we were following an airplane; we didn't interact much more with ground control but when we got the clearance to switch to the tower frequency; it sounded like they called us [flight] Y instead of X. The first officer queried about it being [flight] X not Y; and the controller responded uh; yeah; that's what I meant. Looking back; they didn't sound too sure. Tower cleared us to line up and wait. We both once again thought they said flight Y. As we started to move we thought we better clarify again and just as the first officer was about to ask we received take off clearance. By now; we really felt like something was wrong. The first officer said; 'confirm ...flight X is cleared for takeoff'. Controller says no; flight Y; cancel takeoff clearance. Immediately another voice comes on radio and says no; it's okay you are same direction as flight X; cleared for takeoff. I'm assuming that was the monitor or supervisor since it was a different voice so quickly. We departed uneventfully. Once we got out of sterile; I pulled up to see if there was any flight Y leaving about the same time. Sure enough; flight Y was supposed to leave at about the same time but was delayed by about 15 minutes. I'm not sure where the error chain began or how; but it was finally broken. I'm not writing this to get any controllers in trouble. After all; I'm not sure if any of this was their fault or if it was ours. I just wanted to bring it to your attention to obviously keep us out of trouble but to also try and understand how it could have happened. I keep thinking how it could continue with our transponder on with our code how do we show up as flight Y not X? Did they misunderstand our call sign on first contact or did we miss hear their response? It's not like when another aircraft tries or take your clearance because of similar sounding call signs. Nobody was responding to the calls but us. I also think that while flight Y was supposed to be taxiing out at the same time; because of the delay they were too far behind for it to be an issue for them. We may have been airborne before they were even on ground frequency. Heck; did ground metering accidentally hear flight Y instead of X on initial contact. I just don't know. Anyway; sorry for the long-winded response. Thank you for your time.

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Original NASA ASRS Text

Title: B737 Captain confusion about takeoff clearance due to a similar sounding callsign.

Narrative: We were taxiing out as flight X going. We were cleared to follow a Regional Jet to the runway. I thought the Ground Controller was a little garbled when our call sign was used but the first officer read back the clearance with no problem from ATC. Since we were following an airplane; we didn't interact much more with Ground Control but when we got the clearance to switch to the tower frequency; it sounded like they called us [flight] Y instead of X. The First Officer queried about it being [flight] X not Y; and the controller responded uh; yeah; that's what I meant. Looking back; they didn't sound too sure. Tower cleared us to line up and wait. We both once again thought they said flight Y. As we started to move we thought we better clarify again and just as the First Officer was about to ask we received take off clearance. By now; we really felt like something was wrong. The First officer said; 'Confirm ...flight X is cleared for takeoff'. Controller says no; flight Y; cancel takeoff clearance. Immediately another voice comes on radio and says no; it's okay you are same direction as flight X; cleared for takeoff. I'm assuming that was the monitor or supervisor since it was a different voice so quickly. We departed uneventfully. Once we got out of sterile; I pulled up to see if there was any flight Y leaving about the same time. Sure enough; flight Y was supposed to leave at about the same time but was delayed by about 15 minutes. I'm not sure where the error chain began or how; but it was finally broken. I'm not writing this to get any controllers in trouble. After all; I'm not sure if any of this was their fault or if it was ours. I just wanted to bring it to your attention to obviously keep us out of trouble but to also try and understand how it could have happened. I keep thinking how it could continue with our transponder on with our code how do we show up as flight Y not X? Did they misunderstand our call sign on first contact or did we miss hear their response? It's not like when another aircraft tries or take your clearance because of similar sounding call signs. Nobody was responding to the calls but us. I also think that while flight Y was supposed to be taxiing out at the same time; because of the delay they were too far behind for it to be an issue for them. We may have been airborne before they were even on ground frequency. Heck; did ground metering accidentally hear flight Y instead of X on initial contact. I just don't know. Anyway; sorry for the long-winded response. Thank you for your time.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.