It is a widely held view that when a patient with type I diabetes mellitus and diabetic retinopathy or neuropathy develops renal impairment the renal lesion will be diabetic glomerulonephropathy. This has been extrapolated to apply to type II diabetes. We have performed a retrospective study of the clinical data of patients with diabetes mellitus who have had a renal biopsy between November 1980 and December 1990. Seventy-one patients were biopsied, data were available on 68. Nineteen of 22 type I diabetics had diabetic glomerulopathy, two had diabetic glomerulopathy in addition to another lesion only one patient did not have diabetic glomerulopathy. Twenty-three of 46 type II diabetics had diabetic glomerulopathy alone 22 having an alternative diagnosis. Eight further patients were identified who were not known to be diabetic at the time of renal biopsy, but whose biopsies revealed diabetic glomerulopathy. These data suggest that patients with type II diabetes and renal impairment should have a renal biopsy as part of their investigation.