Narrative:

While at cruise; the a flight attendant called the cockpit and stated that he could smell a strange odor in the cabin. He said the odor was not present on takeoff; but had developed over the last few minutes. I told him to have the other flight attendants check and give input; and to try to make a determination if they thought it was smoke; fumes; or fire. There was no odor in the cockpit; and the report from the cabin was inconclusive; so I told the first officer to put on oxygen while I walked back into the cabin and see if I could smell anything. At first I smelled nothing and the F/as said that it was most noticeable when walking from the back to the front of the aircraft. Upon doing this; I thought that possibly there was a very slight odor just forward of the wings; but even then it was impossible to classify it. None of the crew thought the odor was from an electrical issue; smoke or fire. I thought maybe it could have been something in carry-on luggage; so we opened the bins and checked. We just could not say with any certainty what it was. All four flight attendants said they were concerned about it because they had never smelled anything like it before. I told them that we would most likely be diverting; but that I needed to coordinate with the first officer and dispatch before making a final decision. I returned to the cockpit; we discussed the situation; and got a phone patch with dispatch. We decided to divert. While enroute we ran the smoke; fire or fumes checklist; and diverted. Fire trucks followed us to the gate; and we deplaned all the passengers and their belongings normally.company maintenance was unable to find anything wrong with the aircraft. The discrepancy was signed off and the aircraft returned to service. I contacted the chief pilot on call to review the situation; we reboarded the aircraft and continued to [our destination] with no further issues.

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Original NASA ASRS Text

Title: B737-800 flight crew reported diverting to an alternate after noticing a slight odor in the cabin.

Narrative: While at cruise; the A Flight Attendant called the cockpit and stated that he could smell a strange odor in the cabin. He said the odor was not present on takeoff; but had developed over the last few minutes. I told him to have the other Flight Attendants check and give input; and to try to make a determination if they thought it was smoke; fumes; or fire. There was no odor in the cockpit; and the report from the cabin was inconclusive; so I told the F/O to put on oxygen while I walked back into the cabin and see if I could smell anything. At first I smelled nothing and the F/As said that it was most noticeable when walking from the back to the front of the aircraft. Upon doing this; I thought that possibly there was a very slight odor just forward of the wings; but even then it was impossible to classify it. None of the crew thought the odor was from an electrical issue; smoke or fire. I thought maybe it could have been something in carry-on luggage; so we opened the bins and checked. We just could not say with any certainty what it was. All four Flight Attendants said they were concerned about it because they had never smelled anything like it before. I told them that we would most likely be diverting; but that I needed to coordinate with the F/O and Dispatch before making a final decision. I returned to the cockpit; we discussed the situation; and got a phone patch with Dispatch. We decided to divert. While enroute we ran the Smoke; Fire or Fumes Checklist; and diverted. Fire trucks followed us to the gate; and we deplaned all the Passengers and their belongings normally.Company Maintenance was unable to find anything wrong with the aircraft. The discrepancy was signed off and the aircraft returned to service. I contacted the Chief Pilot on Call to review the situation; we reboarded the aircraft and continued to [our destination] with no further 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.