The effect of risk factors may be either direct, via morbidity, or indirect, via change in the natural course of the disease and increased mortality rate. The effect of some risk factors (including arterial hypertension, coronary heart disease, atrial fibrillation, diabetes, nicotinism, alcoholism, wrong diet) on brain stroke (BS) prevalence was outlined in an overview of the literature. Because of high mortality rates caused by BS in Poland, a prospective epidemiological study was carried out in Warsaw in the years 1991-1992, analyzing the effect of risk factors on mortality rate within 30 days from the onset of BS, and on the probability of survival over the following 12 months. In the sample under study the main risk factor increasing the BS mortality rate within 30 days from onset, besides age, severity of disorientation and neurological symptoms, was chronic spastic bronchitis (with risk increase index of 1.52, p<0.05). On the other hand, chances of survival over the following 12 months were significantly lower in patients with atrial fibrillation, right ventricle insufficiency, or myocardial infarction within 6 months prior to the BS onset.