BACKGROUND: The applicants’ self-declaration of medical history is crucial for safety. Some evidence indicates that under-reporting of medical conditions exists. However, the magnitude in a population of aviation personnel has not been reported earlier.METHODS: A total of 9941 applicants for medical certificate/attestation for aviation-related safety functions during the last 5 yr up to December 2019 were registered at the Civil Aviation Authority Norway. E-mail addresses were known for 9027 of these applicants, who were invited to participate in a web-based survey.RESULTS: Among the 1616 respondents, 726 (45%) were commercial pilots, 457 (28%) private pilots, 272 (17%) air traffic controllers, and the remaining were cabin crew or crew in aerodrome/helicopter flight information service (AFIS or HFIS, respectively). A total of 108 were initial applicants. The age group 50+ constituted the largest proportion of respondents (53%). Aeromedical certification in general was believed to improve flight safety “to a high” or “very high extent” by 64% of the respondents. A total of 188 individuals (12%) admitted having under-reported information related to one or more categories, including mental (3%) or physical health (4%), medications (2%), and drug use, including alcohol use (5%). Among these, 21 participants believed their own under-reporting “to some” or “to a high extent” affected flight safety. In total 50% of noninitial applicants reported that they knew colleagues who had under-reported information. Analyses revealed that being a commercial pilot showed a higher risk for under-reporting compared with other classes and the perception of aeromedical examiners in a supportive or authoritative role reduced the risk.CONCLUSIONS: Under-reporting of medical conditions could be significant in aviation. Further studies should be conducted to investigate the true extent of under-reporting and its impact on flight safety and what mitigating measures might be recommended.Strand T-E, Lystrup N, Martinussen M. Under-reporting of self-reported medical conditions in aviation: a cross-sectional survey. Aerosp Med Hum Perform. 2022; 93(4):376–383.