Parvovirus B19 infection causes chronic anaemia in immunodeficient individuals by selective suppression of erythropoiesis. The bone marrow morphology is characteristic of pure red cell aplasia (PRCA). To determine the frequency of B19-induced PRCA we retrospectively analysed a series of 57 PRCA patients. B19 DNA was present in serum of eight patients (14%) and could be extracted from bone marrow aspirate slides from five of these patients. Recent exposure to the virus was confirmed by the presence of anti-B19 IgM in sera from four and by the finding of giant pronormoblasts in marrow aspirates from five of the B19 DNA-positive patients. The sensitivities of anti-B19 IgM and of giant pronormoblasts were only 50% and 63%, respectively; specificities were 90% and 92%. Unexpectedly, PRCA in two B19 DNA-positive patients remitted after antilymphocyte globulin or cyclosporin A therapy, suggesting that the clinical course of B19-induced PRCA may be indistinguishable from other forms of PRCA. As therapy with immunoglobulin is uniformly effective for treatment of B19-associated anaemia, our data suggest that all patients with acquired PRCA should be evaluated for evidence of B19 infection. B19 DNA analysis is the most reliable method to demonstrate infection.