Proteinuria and microalbuminuria occur with a highly variable severity and are associated with progression of autosomal dominant polycystic kidney disease (ADPKD). Dilazep dihydrochloride, an antiplatelet drug, is effective in patients with immunoglobulin A nephropathy or diabetic nephropathy. We studied whether dilazep dihydrochloride affects the urinary albumin excretion (UAE) in normotensive and hypertensive patients with ADPKD. Twelve normotensive ADPKD patients with microalbuminuria were randomly assigned to two groups: a dilazep (300 mg/day) treatment group (n = 6, group A) and a placebo group (n = 6, group B). In addition, 10 hypertensive ADPKD patients with microalbuminuria were randomly assigned to two groups: a dilazep (300 mg/day) treatment group (n = 5, group C) and a placebo group (n = 5, group D). Treatment with dilazep was continued for a period of 6 months, at the end of which the UAE was reduced form 130 ± 52 to 46 ± 26 µg/min (p < 0.01) in group A. There was no reduction in group C. There were no changes in UAE in placebo groups B and D. These results suggest that dilazep dihydrochloride may be effective in reducing UAE in normotensive ADPKD patients with microalbuminuria.