Diabetic nephropathy is the leading cause of need for kidney dialysis and/or transplant in the United States. Zopolrestat is a new potent reversible noncompetitive inhibitor of aldose reductase. In experimental models of early diabetic nephropathy zopolrestat prevents and reverses several potential important abnormalities of renal function. Phase I studies demonstrated that zopolrestat is well tolerated in healthy male volunteers receiving doses of up to 1200 mg/day for up to 15 days. The purpose of this phase III study is to determine the safety and efficacy of administration of zopolrestat for 52 weeks in normotensive adults with type I diabetes and with incipient nephropathy (stage of microproteinuria).