The study was aimed at further investigating the circadian and circannual patterns of stroke onset. Study design and type of participants: 977 strokes (475 in men and 502 in women) concerning 926 subjects (457 men and 469 women) admitted to Ferrara Hospital in two calendar years (1990-1991), were prospectively investigated. The strokes were classified as based on cerebral infarction (CI), transient ischemic attack (TIA) and cerebral hemorrhage (CH: subarachnoid and intracerebral hemorrhage). Two statistical models of analysis were used. The assessment of circadian and circannual periodicity was performed utilizing the single cosinor method. A separate analysis was performed after distribution of events into 6-hour intervals, and chi-square test for fit was applied to the number of observed versus expected cases. The majority of strokes occurred in the morning between 7 a.m. and noon (35% of cases) and the hypothesis of a uniform distribution of the time onset was rejected on the basis of the chi-square for all subtypes of stroke. A circadian rhythm was found for CI and TIA with acrophase at the 11.56 and 12.41 respectively. Also a circannual periodicity was found for CI with a prevalent peak in October. The spectral analysis detected a circadian cycle for CH having a period of 4 h, and a circannual cycle for TIA with a period of 4 months. This study confirms that stroke is a high-chrono-risk disease, with specific circadian and circannual rhythms. This is very important for a better understanding and control of the underlying factors and in terms of prevention.