This review concerns the acute phase of stroke. It describes incidence, prevalence, etiology, diagnosis and treatment together with the possibilities for prevention. The incidence of stroke in the Danish population is about 2/1000 person years and has been largely unchanged during the last 20 years. About 85 per cent of strokes are caused by cerebral infarcts, ten percent by intracerebral haemorrhages and about five per cent by subarachnoid bleeding. The incidence increases with age. Up till age 65 years the ratio between men and women is two to one, while the ratio in the oldest age group approaches one to one. The most important risk factors for stroke are smoking, arterial hypertension, previous cerebrovascular disease, heart disease and diabetes mellitus. Till now, no treatment has been documented as effective in reducing the cerebral damage caused by acute stroke. Ongoing controlled clinical trials in the acute state of ischaemic stroke are testing the effect of thrombolytic therapy, treatment with calcium antagonists, glutamate receptor antagonists, aspirin and heparin. The general medical treatment including nursing and physiotherapy in the acute phase is described. Within recent years benefit of various strategies of stroke prevention has been documented.