OBJECTIVE Protein restriction delays the progression of non-diabetic and type 1 diabetic renal disorders. This study assessed whether protein restriction delays the onset or early progression of renal disorders in type 2 diabetes. METHODS Randomized controlled trial. Outcomes were albuminuria (mg/24 h) and, as an estimate of the glomerular filtration rate, cimetidine-influenced creatinine clearance. METHODS Primary care. METHODS Patients with type 2 diabetes and microalbuminuria or at least detectable albuminuria, or a diabetes duration >5 y. METHODS The experimental group received dietary counselling on protein restriction (n=63); a control group (n=68) received the usual dietary advice. The duration of intervention and follow-up was 28+/-7 months. RESULTS After 6 months, protein intake differed only by 0.08 g/kg/day between the study groups. Subsequently, this difference decreased and eventually disappeared. An initial effect of protein restriction on albuminuria in favor of the experimental group was not sustained, and the glomerular filtration rate decreased in the experimental group at a 1.6+/-2.2 ml/min/1.73 m(2) y lower rate than in the control group (P=0.5). Comparison of patients in the experimental group with a decrease in protein intake of at least 0.20 g/kg/day, with controls with no decrease, indicated a similarly small and insignificant effect on glomerular filtration rate. CONCLUSIONS It is concluded that, in the longer term prevention or delay of renal damage in patients with type 2 diabetes, protein restriction is neither feasible nor efficacious.