A 44-year-old man, with autosomal dominant polycystic kidney disease and hypertension under satisfactory control, developed nephrotic syndrome with negative serology. Open renal biopsy revealed focal glomerular sclerosis. Prior to the appearance of heavy proteinuria, serum creatinine was 1.7 mg/dl. After the nephrotic syndrome had been established, renal function deteriorated rapidly and hemodialysis was started within 2.6 years. In patients with autosomal dominant polycystic kidney disease, the appearance of nephrotic range proteinuria along with a rapid decline in renal function indicates the presence of a glomerular lesion, which needs to be investigated by renal biopsy.