13 patients with HIV-related immune thrornbocytopenia (HIV-ITP) and 10 patients with chronic idiopathic thrombocytopenic purpura (C-ITP) were treated with a single course ofα-2b-Interferon (IFN 3×106IU sub-cutaneously for 12 d). The patients had platelet counts lower than 40×109/L and thrombocytopenia persisting for over 1 year (range 1–22 years); 7 patients were refractory to previous conventional therapy, 5 were responsive, and 11 had not been previously treated. The response to IFN was complete in 8 patients (platelets>100×109/L). partial in 7 (platelets 50–110×109/L); 8 patients showed no response. The treatment with IFN was stopped after 4 d in one patient due to a fall in platelet count. The maximal platelet count (median peak 116±55 SD×109/L platelets) was obtained after 13.7±2.98d and the improvement in platelet count was maintained for 22.8±8.6 d. No difference in platelets response was observed between HIV-ITP and C-ITP. The response to IFN seems to be related to the one obtained with previous treatments. Indeed 80% of the patients who were responsive to previous steroids, high dose immunoglobulins or azidothymidine (HIV-ITP) showed a complete or partial response while only 43% of the refractory patients showed a partial response; the positive response rate in previously untreated patients was 73%.