Thirty-seven patients with hairy-cell leukaemia were retrospectively reviewed for the presence of autoimmune disease. Ten definite and two probable cases were identified; these patients had positive serologies (immune complexes, antinuclear antibodies or rheumatoid factor) or biopsy-proven vasculitis. Clinically, two distinct syndromes were recognized. Six patients had joint symptoms, usually associated with nodular skin lesions; all responded promptly to therapy. Four additional cases had a more severe disease consisting of fevers, malaise, weight loss, skin rash, and variable visceral involvement; there was one death in this group. There appeared to be no relationship between presence of vasculitis and the severity or progression of the underlying malignant disease. We conclude that autoimmune disease is much more frequent in hairy-cell leukaemia than has previously been recognized, and that the outcome in these syndromes is usually good. Although the autoimmune syndrome generally responds promptly to splenectomy, corticosteroids, or cytotoxic therapy, failure to recognize this complication may lead to increased morbidity and occasional mortality.