The authors analysed seasonal and monthly incidence of and mortality from ischaemic stroke (IS) in the region of Białystok (North-Eastern Poland). 839 cases of IS (437 men and 402 women), aged 22 to 94 years, were hospitalised in the Department of Neurology, Medical Academy in Białystok during the analysed period (1990-1997). Significant seasonality in IS incidence was observed in both sex groups, with a nadir in summer. The lowest occurrence of infarcts was observed in August in women, and in June in men. The peak month for IS, independently of sex, was January. 240 patients (28.6%) died of stroke or its complications during the analysed period. The occurrence of fatal IS followed also a clear seasonal pattern with peak in autumn. The authors attempt to explain this seasonal incidence and mortality pattern of IS in relation to variation in temperature, diet, way of life (holidays in summer), and biochemical blood changes, which occur in different seasons of the year.