INTRODUCTION: Commercial aircrews are exposed to potential occupational hazards and, recently, epidemiological studies have examined their morbidity and mortality relative to the general population. Aircrews are, however, likely to differ from the general population in several respects which may affect the validity of such comparisons. METHODS: A cohort of 17,990 commercial aircrews was identified through the United Kingdom (UK) Civil Aviation Authority medical records and is being followed for morbidity and mortality. Demographic, lifestyle, reproductive, and medical characteristics of commercial aircrews were compared with those of: 1) UK air traffic control officers (ATCOs; N = 3386) identified in a similar way as aircrews; and 2) estimates for the UK general population. RESULTS: Aircrews and ATCOs had similar characteristics, except that sex-age-adjusted prevalences for current smoking, obesity, and hypertension were statistically significantly higher in the latter. Both aircrews and ATCOs differed considerably from the general population with, for instance, much lower sex-age-adjusted prevalences of current smoking, obesity, and hypertension but higher levels of regular physical exercise. Age-adjusted fertility rates among female aircrews and ATCOs were only one-third of those in the general population. These differences were slightly attenuated when comparisons with the general population were restricted to its highest socio-economic stratum. DISCUSSION: The differences between aircrews and the general population are consistent with a strong "healthy worker effect." Aircrews and ATCOs undergo a similar employment selection process and thus taking the latter as the reference population, in addition to the general population, will help to minimize the "healthy worker effect" and gain insight into its biases.