BACKGROUND The degree of proteinuria in patients with diabetes correlates strongly with both an increase in progression of nephropathy as well as cardiovascular events. Moreover, post hoc analyses of recent clinical trials support the concept that reductions of blood pressure and proteinuria correlate with a slowed progression of nephropathy. Both angiotensin converting enzyme (ACE) inhibitors and the nondihydropyridine calcium antagonists, (non-DHPCAs) reduce both arterial pressure and proteinuria in those with diabetic nephropathy. METHODS The present randomized, open label, parallel group designed study tests the hypothesis that, at similar levels of blood pressure, the combination of an ACE inhibitor, trandolapril (T) with the non-DHPCA, verapamil (V) produces a greater reduction in proteinuria over either agent alone at one year. Thirty-seven participants, mean age 59.6 +/- 5.8 years, with nephropathy (baseline creatinine 1.4 +/- 0.3 mg/dl and proteinuria of 1342 +/- 284 mg/dl) secondary to type 2 diabetes completed the study. Doses of drug were titrated in each group over eight weeks to achieve a goal blood pressure of < 140/90 mm Hg. All participants were counseled to ingest a sodium diet of < 120 mEq/day. RESULTS Proteinuria reduction from baseline was significantly greater in the T+V group compared to either T alone (-33 +/- 8%, T vs. -62 +/- 10%, T+V; P < 0.001) or V alone (-27 +/- 8%, V vs. -62 +/- 10%, T+V; P < 0.001). No significant differences in either glomerular filtration rate, arterial pressure, fasting blood glucose or urinary sodium excretion were noted at one year. The mean daily dose of the individual components of T+V (2.9 +/- 0.8 mg, T/219 +/- 21.1 mg V) was significantly lower than the dose of either T alone 5.5 +/- 1.1 mg/day (P < 0.01) or V alone 314.8 +/- 46.3 mg, given in two divided doses (P < 0.01). CONCLUSIONS These data support the concept that the combination of an ACE inhibitor with a non-DHPCA reduce proteinuria to a greater extent than either agent alone. This added antiproteinuric effect occurs at lower doses of each drug and is independent of further reductions in arterial pressure. These findings could have ramifications for slowing renal disease progression in patients with nephropathy from type 2 diabetes.