Eleven of 30 patients with MCTD, followed for a mean of 10 years, developed immune complex nephropathy (five membranous, two mesangial, one mixed, and one sclerosing) with NS in nine of 11. Another patient had membranous nephropathy at autopsy. Patients with renal disease tended to have more systemic manifestations than those without. NS was at times of abrupt onset, recurrent, and/or persistent. Anti-RNP and serum complement were not helpful in predicting nephritis. Seventy-two percent of nephropathy and 62% of NS episodes resolved or improved after corticosteroid therapy. Five patients became hypertensive, two developed chronic renal failure and required chronic dialysis, and one needed acute dialysis twice. One patient progressed to focal proliferative crescentic nephritis with necrotizing arteritis. Three patients with nephropathy died, two of pulmonary hypertension with acute cor pulmonale and one of overwhelming sepsis. Nephropathy is relatively common in MCTD, is associated with substantial morbidity, and with the risk of hypertension and chronic renal failure.