In all of four patients with African trypanosomiasis, thrombocytopenia was present on admission to hospital or developed during the course of the illness. One patient with severe thrombocytopenia died following gastrointestinal haemorrhage shortly after admission to hospital. Kinetic studies in the other three patients showed marked pooling of platelets in the spleen in all, but severe shortening of platelet life-span in only one. Evidence of disseminated intravascular coagulation (DIC) was found in 3 patients, 2 of whom received heparin therapy. These findings provide evidence that thrombocytopenia is a feature of African trypanosomiasis and is due mainly to hypertrophy of the reticuloendothelial system which accompanies the infection. In some patients immune damage to platelets or platelet consumption as part of DIC may be an additional factor contributing to the thrombocytopenia.