Seasonal periodicity in the onset of spontaneous intracerebral hemorrhage was studied in 118 consecutive cases occurring during a six-year span. The patients were urban residents of eastern Minnesota, a region characterized by wide seasonal fluctuations in daylight and temperature. The greatest number of cases consistently occurred each year during January and February. Circannual (about one year) periodicity was demonstrated by statistical analysis using rhythmometric techniques. This periodicity coincided with that reported for mortality from cerebrovascular and cardiovascular diseases in the United States and elsewhere. Available information suggests that in populations at high risk for vascular diseases, climatic conditions might act as synchronizers to endogenous rhythms influencing the periodic occurrence of pathological vascular events.