An epidemiologic retrospective study was conducted from January 1984 to December 1988 on subjects admitted into the Institute of Semeiotica Medica of the University of Ferrara (as a general medicine, unselective department) and presenting a stroke. Year, month and hour of onset of symptoms in 141 patients (all resident in Ferrara, mean age 74.3 +/- 8.3 years) were carefully recorded. The definition of precise clock hour was reached only in 108 cases. The data were then analyzed by means of Halberg's single cosinor. Time series analysis showed a seasonal pattern of disease onset, characterized by a circannual rhythm with acrophase in late winter - early spring (-83.9 degrees). A significant circadian rhythm was found too, with acrophase at afternoon (-242.8 degrees).