Severe kidney enlargement was observed in 6.3% (5 of 80) of goldfish collected from a heavily polluted industrial basin. Externally the fish had generalized swelling and abdominal distension. The kidneys contained numerous large, clear, fluid-filled cysts (polycystic) that ranged in size from microscopic to 4 cm in diameter. Affected kidneys had a wide range of histological changes-including the presence of large multiple cysts that caused severe distension and compression of normal renal tissue, multifocal granulomas, and signs of early, proliferative glomerulonephritis. The histology of affected kidneys is compared with other goldfish from Hamilton Harbour, and with goldfish collected from an alternate site (a population apparently free of polycystic kidney enlargements). This afflication is rare in feral fish populations, and its occurrence in a deteriorated environment such as Hamilton Harbour may provide further evidence of a link between fish health and environmental quality.