To determine whether increased numbers of circulating endothelial cells, a possible indicator of endothelial injury, are present in subjects with sickle cell disease, we measured circulating endothelial cells in 30 normal subjects and in 23 subjects with sickle cell anemia. Mean circulating endothelial cells were significantly higher (P < 0.025) in the sickle cell subjects than in the normal subjects. Circulating endothelial cells were significantly higher than normal in 10 sickle cell subjects studied during painful crisis (P < 0.01) but not in 13 sickle cell subjects studied while in the steady state. To control for the known stimulatory effect of cigarette smoking on circulating endothelial cells, we analyzed the results for smokers and nonsmokers separately. Mean circulating endothelial cells were not significantly higher in sickle cell subjects who smoked (n = 10) than in normal subjects who smoked (n = 8), but were significantly higher (P < 0.05) in sickle cell nonsmokers (n = 13) than in normal nonsmokers (n = 22). Among nonsmoking sickle cell subjects, mean circulating endothelial cells were significantly higher than normal (P < 0.01) during painful crisis (n = 7), but not in the steady state (n = 6). We conclude that circulating endothelial cells are significantly increased in sickle cell crisis, and may indicate the occurrence of acute endothelial injury during episodes of microvascular occlusion.