4 patients had typical features of thrombotic thrombocytopenic purpura (TTP) after 3 to 8 weeks of ticlopidine therapy. In 2 ticlopidine was the only medication taken before TTP; in another, rechallenge with drugs other than ticlopidine that the patient had been taking did not produce relapse. In all patients the delay between start of ticlopidine and TTP was in the same range as the latent period before ticlopidine-induced neutropenia. No relapse occurred in the 4 to 43 months of follow-up in the 3 surviving patients.