We describe two patients who presented with mixed connective tissue disease. Both had antinuclear antibodies and antibodies to the extractable nuclear antigen U1RNP, and both were found to be seropositive for HTLV-1 infection. We found no evidence of HTLV-1 infection in 20 other patients known to have antibodies to U1RNP or in 36 British and West German patients with idiopathic adult polymyositis. In addition the serum from 20 anti-HTLV-1 positive patients did not contain antinuclear antibodies or antibodies to U1RNP. We conclude that rheumatological disease is associated with HTLV-1 through geographical rather than aetiological means.