Some epidemiologic features of the painful crisis in homozygous sickle cell disease were examined in a retrospective study of 995 painful crises. Previously reported associations with cold weather and pregnancy were confirmed. There was a striking increase in painful crises in male patients between the ages of 15 and 25 years, whereas female patients showed little age-related change. The frequency of painful crises correlated positively with hemoglobin levels and reticulocyte counts in both sexes and negatively with mean corpuscular volume in female patients. There was a striking increase in painful crises in male patients with hemoglobin levels above 8.5 g/dL (greater than 85 g/L). High hemoglobin levels appear to be an important risk factor for painful crises.