Narrative:

Approaching ord on the WATSN3 arrival we were assigned a speed of 250 kts and told to expect the ILS 27L. After passing voglr on the arrival we were given a turn to heading 250 to intercept the ILS 27L. The radios were very busy and at this point we were north of the localizer and planned to ask ATC what their intentions were; but this vector to intercept made sense. We were at (I think) 9;000 ft at this time. After intercepting we were given a descent to 7;000 ft and a short time later we were told that we were cleared for the approach and slow to 170 kts. We were around nrmah at this point which has an altitude constraint of 9;000 ft so I queried ATC to make sure it was okay to go down to 7;000 ft and then comply with the approach plate altitudes. They told us that was what they expected.since we were at 250 kts and descending the PF (the first officer) disconnected the autopilot to get the airplane to slow down faster. We could see that the speed reduction was because of an aircraft about 3 1/2 miles ahead; so the PF was doing his best to slow. ATC then asked us to slow to 160 kts. We continued to slow as quickly as possible; extending the landing gear and flaps all the way to 30 - our final configuration. ATC then told us to cancel our approach clearance and maintain 6;000 ft. I acknowledged the clearance; dialed it in the altitude window; and the PF confirmed the altitude. At this time I was busy backing up the PF and watching the spacing on the traffic ahead both visually and on TCAS. The next thing I hear is the PF saying we have gone through the 6;000 ft altitude and I see 5;600 ft on the altimeter. As soon as the PF noticed the deviation he started correcting back to 6;000 ft. ATC queried us about our altitude - then told us to continue our descent to 5;000 ft. We got our spacing on the traffic ahead of us and ATC recleared us for the approach. We continued the approach and landed without any other issues.

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Original NASA ASRS Text

Title: B737NG flight crew reported an altitude overshoot occurred on approach to ORD during a time of high workload and distractions.

Narrative: Approaching ORD on the WATSN3 arrival we were assigned a speed of 250 kts and told to expect the ILS 27L. After passing VOGLR on the arrival we were given a turn to heading 250 to intercept the ILS 27L. The radios were very busy and at this point we were north of the localizer and planned to ask ATC what their intentions were; but this vector to intercept made sense. We were at (I think) 9;000 ft at this time. After intercepting we were given a descent to 7;000 ft and a short time later we were told that we were cleared for the approach and slow to 170 kts. We were around NRMAH at this point which has an altitude constraint of 9;000 ft so I queried ATC to make sure it was okay to go down to 7;000 ft and then comply with the approach plate altitudes. They told us that was what they expected.Since we were at 250 kts and descending the PF (the FO) disconnected the autopilot to get the airplane to slow down faster. We could see that the speed reduction was because of an aircraft about 3 1/2 miles ahead; so the PF was doing his best to slow. ATC then asked us to slow to 160 kts. We continued to slow as quickly as possible; extending the landing gear and flaps all the way to 30 - our final configuration. ATC then told us to cancel our approach clearance and maintain 6;000 ft. I acknowledged the clearance; dialed it in the ALT window; and the PF confirmed the altitude. At this time I was busy backing up the PF and watching the spacing on the traffic ahead both visually and on TCAS. The next thing I hear is the PF saying we have gone through the 6;000 ft altitude and I see 5;600 ft on the altimeter. As soon as the PF noticed the deviation he started correcting back to 6;000 ft. ATC queried us about our altitude - then told us to continue our descent to 5;000 ft. We got our spacing on the traffic ahead of us and ATC recleared us for the approach. We continued the approach and landed without any other issues.

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.