To assess possible age-related differences in the time of occurrence (hour of the day, day of the week, month of the year) of cardiopulmonary collapses of presumed cardiac etiology, 3,305 out-of-hospital patients registered by the Belgian Cardiopulmonary-Cerebral Resuscitation Study Group between 1983 and 1990 were studied. Occurrence of call-time was separately tabulated for patients less than versus above 60 years of age and tested for differences with the Kolmogorov-Smirnov two-sample test. Only the circadian patterns of all out-of-hospital cardiac arrests of presumed cardiac etiology showed a significant difference between the two age-groups; the prominent morning peak (6 a.m.-10 a.m.) in incidence was less pronounced in the younger age-group. The overall incidence is lower in the summer and during mid-week. These trends are similar for both age-groups.