Platelet volume distribution was examined in 47 thrombocytopenic subjects with a platelet count between 20 109 and 150 109/1. Thrombocytopenia was attributed to defective production in 19 cases (12 acute leukemia) and to accelerated destruction in 28 cases (27 idiopathic thrombocytopenic purpura). Six volume variables were retained from the platelet volume distribution curves. A multivariate discriminant analysis has shown that the median of the curves was the most discriminant parameter between the two types of thrombocytopenia: the value of the median was less than 7 μ m3 in 89.5% of patients with defective platelet production and greater than 7 μ m3 in 85.7% of those with excessive platelet destruction. Thus, platelet volume measurement and the median value in particular allows a rapid orientation of the etiological diagnosis of thrombocytopenia.