Thirty-eight patients with chronic refractory idiopathic thrombocytopenic purpura (ITP) were treated with weekly slow infusions of vincristine (0.02 to 0.04 mg/kg) or vinblastine (0.1 to 0.2 mg/kg). Twenty-two patients showed good to excellent responses after one to eight infusions. These responses were generally short, and lasted only in six patients after discontinuance of the therapy. The efficacy was comparable between vincristine and vinblastine. Neither the age, sex, duration of the disease, prior splenectomy nor combined use of adrenocortical steroids was likely to have influenced the therapeutic effect. Side effects such as peripheral neuropathy, alopecia, gastrointestinal symptoms and leukopenia occurred in 34 patients, and necessitated discontinuance of the therapy in eight patients. Slow infusions of vinca alkaloids can be an effective means of inducing platelet response in patients with chronic refractory ITP, but frequent side effects limit its clinical usefulness.