Mixed connective tissue disease (MCTD) is a syndrome with features of scleroderma, rheumatoid arthritis, SLE, and polymyositis-dermatomyositis. The manifestations of the disease include arthritis, Raynaud's phenomenon, scleroderma of the fingers, muscle weakness and tenderness, interstitial lung disease, and a skin rash resembling either dermatomyositis or SLE. These patients have antibodies to U1 snRNP in high titer. Renal disease is unusual in these patients. The disease appears to respond to moderate doses of corticosteroids.