The benefits of angiotensin-converting enzyme inhibitors and angiotensin II (ATII) receptor antagonist therapy of diabetic nephropathy (DNP) are thought to be largely the result of attenuation of ATII effects on proteinuria. The aim of the study was to ascertain whether there is the additive anti-proteinuric effect of enalapril plus losartan in DNP. Twenty-two patients with DNP were studied. Patients were randomly assigned to enalapril 10 mg/day (11 patients) or losartan 50 mg/day (11 patients) administered in a single oral dose in the morning for 12 weeks. and then, in 10 patients (five patients from enalapril group and five patients from losartan group), combination therapy (10 mg/day enalapril and 50 mg/day losartan) was started and continued for 12 weeks. In 12 patients, initial drugs dosages were doubled (six patients 20 mg/day enalapril and six patients 100 mg/day losartan), and monotherapy was continued for 12 weeks. Blood pressure and proteinuria were measured before and after therapy. Adverse effects were recorded at every visit. Proteinuria decreased by 33% with enalapril and losartan administered alone (p < 0.05). Co-administration of enalapril and losartan decreased proteinuria by a greater extent compared with enalapril and losartan administered alone (51%, p < 0.05). This proteinuria level was significantly lover than the proteinuria level of 12 weeks therapy with enalapril and losartan alone. The decrease of proteinuria was 37% in double-dose monotherapy group (p < 0.05). Reduction of mean arterial blood pressure (MAP) in co-administration of enalapril and losartan was higher than enalapril and losartan administered alone (p < 0.05). Combination of enalapril and losartan decreased proteinuria and MAP by a greater extent compared with enalapril and losartan administered alone. We have found that proteinuria reduction induced by combined therapy is maintained throughout short-term follow-up; a greater anti-proteinuric response was observed in the patients with DNP.