STUDY OBJECTIVES: The literature has demonstrated circadian rhythms in the occurrence of nonfatal myocardial infarction, ischemia, and sudden death. We hypothesized that requests for helicopter transport of acutely ill cardiac patients followed a similar circadian pattern and differed significantly from requests for helicopter transport of other categories of patients. DESIGN: Prospective study of requests for helicopter transport of 1,128 consecutive air medically transported patients over a 24-month period. SETTING: One tertiary-care teaching hospital. MEASUREMENTS AND MAIN RESULTS: The periodic structure of the time distribution of cardiac requests for helicopter transport was examined with a two-harmonic regression analysis using a 24-hour period of oscillation. Seven hundred eighty-seven cardiac and 315 noncardiac patients could be evaluated. The times of requests for helicopter transport were tabulated into hourly intervals. Cardiac-related requests for helicopter transport were significantly different from noncardiac-related requests for helicopter transports (P less than .009 by Wilcoxon rank sum test, P less than .032 by Kolmogorov-Smirnov test). The regression model for cardiac requests for helicopter transport was also significant (P less than .0001, R2 = .81) with increasing requests for helicopter transport from 6:00 AM until 12:00 noon. CONCLUSION: The time distribution of requests for helicopter transport for cardiac patients demonstrates a striking circadian variation not observed in noncardiac patients. This observation strengthens mechanistic inferences from studies of circadian variation and suggests a "morning-loaded" staffing pattern for air medical services predominantly transporting cardiac patients.