Chronic idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder in which activated T-helper (Th) cells and different Th-cell cytokines might play an important role. We have recently reported that chronic ITP patients in remission had elevated plasma levels of the Th3 cytokine transforming growth factor-beta1 (TGF-beta1), possibly as a part of a bystander immune suppression. In the present study we found that, in ITP patients with active disease [platelet count (plc) < 50 x 10(9)/l], mitogen-stimulated peripheral blood mononuclear cells (PBMC) had a significantly reduced production of TGF-beta1 (444 +/- 178 pg/ml; n = 6) compared with patients with plc 50-150 x 10(9)/l (1293 +/- 374 pg/ml; n = 9; P < 0.05), patients with plc >150 x 10(9)/l (1894 +/- 244 pg/ml; n =12; P <0.005) and healthy controls (1698 +/- 241 pg/ml; n = 10; P < 0.01). Nineteen per cent of ITP patients expressed a platelet-induced PBMC proliferation. Surprisingly, 22% of the ITP patients had a PBMC proliferation below the normal range, i.e. a suppressed proliferation in the presence of platelets; five of these six patients had active disease. In summary, this study demonstrated that chronic ITP patients with active disease had reduced PBMC production of the Th3 cytokine TGF-beta1. This result gives further support to the theory that chronic ITP in active phase is associated with a downregulated Th3-response.