Intravenous (i.v.) infusions of Ig concentrates are an effective but expensive treatment for patients with autoimmune thrombocytopenic purpura (AITP). The optimal treatment protocol and the long-term results are uncertain, and the precise mechanism by which the platelet count increases is poorly understood. Twenty adult patients with chronic AITP were enrolled in a prospective study to compare the respective efficacy of two high-dose IVIgG induction regimens (1 g v 2 g/kg body weight) and the long-term effect of six 1 g/kg body weight i.v. IgG reinfusions. An initial response was observed in all 18 evaluable patients: the platelet count increased to a mean value of 251 x 10(9)/L (range 72 to 836 x 10(9)/L) and the mean pretreatment platelet count was multiplied by 14.6. No difference in efficiency was observed between the two i.v. IgG dosages. The degree of the platelet count increment correlated in both groups with the increase in the clearance of antibody-coated red blood cells, measured by an isotopic method, but not with the serum IgG elevation. Treatment was considered to have failed in 11 patients, 90 days after the last i.v. IgG reinfusion (D90), because the platelet counts were comparable with pretreatment values. In contrast, a complete response was observed at D90 in five patients (mean platelet count: 184 x 10(9)/L; range: 150 to 250 x 10(9)/L) and a partial response at D90 was obtained in the remaining two patients (platelet counts: 70 and 104 x 10(9)/L). Five of the 7 responders at D90 kept a platelet count above 50 x 10(9)/L during the entire follow-up period (mean 33 months; range: 5 to 66) with no further treatment; unfortunately, no clinical or biologic criteria were found to be predictive of the long-term response. This study shows that an i.v. IgG infusion regimen of 1 g/kg body weight could safely replace the classical 2 g/kg body weight dosage, at least in patients who do not have life-threatening thrombocytopenia. Moreover, repeated i.v. IgG reinfusion could be an alternative for AITP patients in whom splenectomy is contraindicated.