Two patients with autosomal dominant polycystic kidney disease (ADPKD) and concurrent glomerulonephritis are described. Both developed nephrotic-range proteinuria and one showed a concomitant acceleration in the rate of decline of renal function. Subsequent open renal biopsy revealed membrano-proliferative type-1 and mesangio-proliferative glomerulonephritis, respectively. Nephrotic-range proteinuria in the presence of ADPKD, with or without an accompanying decline in renal function, should prompt further investigation to exclude coexisting glomerular disease.