The aim of this study was to define the characteristics of patients with idiopathic thrombocytopenic purpura (ITP) and breast cancer and discuss the relationship between these two diseases. Ten patients treated for breast cancer and presenting with ITP were screened for this study. The diagnosis of breast cancer was confirmed by biopsy or surgical sample. The diagnosis of ITP was defined by 1) platelet count less than 140.10(9)/l with normal or increased number of megakaryocytes on bone marrow aspirate, 2) after exclusion of thrombocytopenia-induced medication or disorders, and 3) absence of splenomegaly. ITP was diagnosed before breast cancer in three cases, concomitantly in three, and after the diagnosis of breast cancer in four cases. Platelet count and breast cancer showed an independent course in seven cases, and appeared to be correlated in the other three patients. No correlation was found between the development of ITP and tumor characteristics. In contrast, the median platelet count was 15.10(9)/l (range 3-26) for the 3 patients with a correlation between the course of ITP and breast cancer evolution and 70.10(9)/l (range 20-90) for the other cases (p = 0.05, Mann-Whitney U test). Breast cancers are associated with ITP, with a parallel course of the two diseases in one third of cases. This may suggest tumor-induced immunologic thrombocytopenia.