A population of B cells with characteristics of chronic lymphocytic leukaemia was found in the peripheral blood of four patients who presented with cutaneous infiltration of atypical CD4+ T cells with cerebriform nuclei. The B cells had a low density of immunoglobulin on their surface membrane, expressed CD5-positivity, and showed monoclonality based on the restriction to either kappa or lambda light chains. In one patient with tumourous pleiomorphic CD4+CD30- T-cell lymphoma of the skin, it was the first manifestation of a concomitant B-cell non-Hodgkin's lymphoma of low-grade malignancy. In three other patients with reactive atypical T-cell erythroderma, there was no evidence for the coexistence of a B-cell malignancy. The number of CD5+ B cells decreased in two erythroderma patients with clinical remission of the cutaneous lesions. It is speculated that the presence of a monoclonal B cell population in patients with T-cell disorders of the skin is due either to a reactive process possibly conferring some protective effect, or a response to an unknown stimulus produced by the T cells.