We measured serum and urinary concentrations of type IV collagen by radioimmunoassay (RIA) in patients with renal diseases and compared them with normal levels. In diabetics with macroproteinuria or with renal insufficiency, serum and urinary concentrations of type IV collagen were higher than in diabetics without nephropathy or with early renal damage as determined by the presence of microproteinuria. Serum concentrations of type IV collagen in patients with membranous nephropathy (MN) showed a significant increase compared with normal controls. Urinary concentrations of type IV collagen in patients with MN, minimal change nephrotic syndrome at the remission phase and nephrotic phase and IgA nephropathy showed a significant increase compared with normal controls. These increases may indicate that a change in basement membrane metabolism has occurred in these renal diseases.