The aim of the study was to verify if sudden cardiac death presents a circadian, circaseptan, and circannual periodicity in its incidence. For the study of distribution of sudden cardiac death by hour of the day, the day of the week, and month of the year, 269 cases (161 males and 108 females), that occurred during the years 1970 through 1987 were considered. Sudden cardiac death, that is, death within 1 hour of onset of symptoms, occurs with greater frequency in the morning hours, on the days from Saturday to Monday, and in the months from October to January. The rhythmometric analysis by "single cosinor" demonstrates significant (p less than 0.05) rhythms, with acrophases at 5:00 a.m., on Monday, and in November. No significant differences (p more than 0.05) were found between male and female groups. These data suggest that sudden cardiac death is an event with high "chronorisk" that is a phenomenon with a greater probability of occurring in a definite period of time. The periodicity in the incidence of sudden cardiac death may be due to relationships between exogenous factors and endogenous biological rhythms.