Peripheral T helper subsets in a patient with acquired pure red cell aplasia (PRCA) and thymoma were analysed by flow cytometry. Thymectomy resulted in a transient but definite improvement in anaemia. A continuous remission was attained by cyclosporin A (CsA) therapy. The changes in Th1/Th2 ratios corresponded with the haematological changes. Specifically, Th2 type lymphocytes were dominant during a haemopoietic suppression and elevation of the Th1/Th2 ratio was concomitant with haematological improvement. These observations suggest that polarization of Th subsets to Th2, induced by clonally expanded (gammadelta-type) T cells in thymoma, are involved in the pathogenesis of PRCA.