From 1973 to 1986 we splenectomized 181 patients with chronic ITP after platelet kinetic studies with 51Cr or 111In. Mean age at diagnosis was 34 (range 4–79 yr). Follow-up of at least 1 yr after splenectomy was available in every patient. 141 patients (78%) achieved remission (platelets > 100 times 109/1 by 3 months after splenectomy), of whom 9 subsequently relapsed. Among the 40 non-responders at 3 months, 3 achieved a later remission spontaneously. Factors associated with response to splenectomy included a high post-operative platelet count (p = 0.0001), younger age at the time of surgery (p = 0.0077) and predominantly splenic sequestration of platelets (p = 0.0002), the two latter factors being partially correlated. In a multivariate analysis, however, only post-operative platelet count and age retained an independent prognostic significance, whereas the sequestration site of platelets had only borderline value. These results are discussed in the context of indications of platelet kinetic studies in chronic ITP, before splenectomy is considered.