In vivo circulating platelet aggregates (CPA) were evaluated in 18 patients aged 6 to 17 years with sickle cell disease and in 11 age and sex matched normal subjects. Twelve patients with sickle cell disease were in steady state and 6 had vaso-occlusive crises. CPA in patients in steady state were similar to those in normal subjects (mean 6 ± 1 % compared to 5 ± 2 %, respectively), whereas patients with vaso-occlusive crisis in acute state had significantly higher CPA (mean 39 ± 8 %) than patients in steady state or normal control individuals (both p < 0.001). CPA decreased in patients with vaso-occlusive crisis (mean 11 ± 4 %) on the tenth day, in association with clinical improvement. This study suggests that in vivo platelet aggregate formation activity, although normal in sickle cell disease patients in steady state, is significantly increased in patients with vaso-occlusive crises.