A case of Behçet's disease with IgA nephropathy was reported in a 17 year old man. He presented with microscopic hematuria which was pointed out by urinary mass screening for school children at age of 14. As he was subsequently evolved proteinuria, a renal biopsy was performed at age of 16. The specimen showed diffuse proliferation of mesangial cells. Immunofluorescent studies showed granular deposition of IgA. At age of 17, he showed typical symptoms of Behçet's disease, namely aphthous stomatitis, perianal ulcers, erythema nodosum-like lesions, uveitis and genital aphtha. Furthermore he showed an increasing of proteinuria and an impairment of his renal function. Serum IgA level and immune complexes during the exacerbation stage were elevated. The uveitis markedly improved with ciclosporin A therapy. Reduction of proteinuria and improvement of renal function were simultaneously observed. These findings may suggest that IgA nephropathy and Behçet's a disease have common immunological pathogenesis, including circulating immune complexes.