A case of diffuse mesangial sclerosis (DMS) associated with Kawasaki disease is reported. A previously healthy Japanese girl, aged 4 months, presented with clinical features of Kawasaki disease. At week 10 of the illness, she developed the nephrotic syndrome, which was refractory to steroid therapy. Renal biopsy demonstrated a diffuse mesangial proliferative glomerulonephritis with microcystic tubular dilatation and, ultrastructurally, marked thinning of the lamina densa in the glomerular basement membrane (GBM) and the tubular basement membrane (TBM) of the proximal tubule. She went into chronic renal failure and died at the age of 11 months. At autopsy, the kidney revealed DMS. Histologically, we found Finnish microcystic disease in its early stages in the biopsy. Using a newly developed monoclonal antibody, we analysed the alpha chains (alpha 1-alpha 6) of type IV collagen in the GBM and TBM. There was no defective constitution of alpha chains on the thin GBM, but the thin TBM of the microcystic proximal tubule showed a weak or discontinuous reactivity for alpha 1 and alpha 2 chains, suggesting faulty formation of the basement membrane. The sclerosing glomeruli of the DMS did not depend on collapse of the GBM, which was positive for alpha 3-alpha 5 chains, but mainly on the proliferation of mesangial matrix, which was positive for alpha 1 and alpha 2 chains.