Blood pressure was measured in 490 non-proteinuric Pima Indians from the Gila River Indian Community in Arizona at least 1 year before the diagnosis of Type 2 (non-insulin-dependent) diabetes mellitus. Urine albumin concentration was measured in the same subjects 0-24 years (mean 5 years) after diabetes was diagnosed. Prevalence rates of abnormal albumin excretion (albumin-to-creatinine ratio > or = 100 mg/g) after the onset of Type 2 diabetes were 9%, 16%, and 23%, respectively, for the lowest to highest tertiles of pre-diabetic mean blood pressure. When controlled for age, sex, duration of diabetes and pre-diabetic 2-h post-load plasma glucose concentration, higher pre-diabetic mean blood pressure predicted abnormal urinary excretion of albumin after the onset of diabetes. This finding suggests that the higher blood pressure seen in diabetic nephropathy is not entirely a result of the renal disease, but may precede and contribute to it.