Renal involvement in mixed connective tissue disease. Report of 5 cases.

5 cases with the compatible serological criteria of mixed connective tissue disease described earlier are presented. In 1 of them with a moderate degree of proteinuria, the renal biopsy disclosed membranous nephritis. However, despite the absence of overt clinical renal disease in the other 4 cases, biopsies disclosed membranous nephritis in 1 and mild mesangial proliferative glomerulonephritis in the remaining 3 cases. In the follow-up of these 4 cases, 2 subsequently developed abnormal urinalysis. Electron microscopic examinations demonstrated electron-dense deposits in glomeruli, and 4 of these patients also had microtubular structures in the endothelial cytoplasm. Contrarily to the original concept, our findings suggest that mixed connective tissue disease also induces immune complex disease.