Clinical and laboratory abnormalities and the presence of atrial fibrillation on admission were examined in relation to hospital mortality in a retrospective study of the 320 patients with acute stroke admitted to a medical unit in a 5-year period. Of clinical factors, only increasing age and the presence of coma were associated with mortality. Atrial fibrillation, present in 25% of all patients, was associated with an increased mortality in patients aged 60-79 years (67% vs 44%, p less than 0.01). In patients under 75 years the mortality associated with raised haematocrit (0.50 or more), present in 11% of all patients, was more than twice as high as it was in those with lower haematocrit (71% vs 31%, p less than 0.005). Other laboratory variables associated with a high mortality were increased levels of mean red cell volume, white cell count, erythrocyte sedimentation rate, globulin, and blood urea and creatinine; and decreased levels of albumin. The high prevalence of and increased mortality associated with atrial fibrillation and raised haematocrit in stroke patients indicate the need for further studies of stroke prevention and treatment in these patients.