We present a retrospective analysis of 111In-platelet sequestration studies in 111 patients with the clinical diagnosis of idiopathic thrombocytopenic purpura (ITP). Fifty-one of these patients underwent splenectomy, independent of the results of the 111In-platelet studies to determine if these isotopic results could accurately predict a beneficial response to splenectomy. Between January 1984 and June 1990, 111 patients who presented with ITP were subjected to a study of autologous 111In-labeled platelets through autotransfusion. The platelet sequestration site was splenic (81%), mixed (12%), or hepatic (7%). Fifty-one patients with persistent drug-resistant thrombocytopenia underwent splenectomy regardless of the isotopic results: 33 patients beyond 6 mo after diagnosis and 18 with high hemorrhagic risks before this delay. The follow-up median duration was 2.9 yr. Thirty-three of the 38 patients with splenic sequestration showed a normalized platelet count, as opposed to 2 of the 13 with mixed or hepatic sequestration (p < 0.001). In addition, platelet survival extended beyond 8 days in six patients, with no apparent sequestration site. The platelet isotopic study performed with this technique appears to be indicated in ITP: it guides clinicians in their decision to perform splenectomy and relates to a more central mechanism certain thrombocytopenias that are inappropriately categorized as ITP.