A detailed review of the microscopic features of 80 nephrogenic adenomas of the urinary tract was undertaken. The lesions occurred in patients who ranged in age from 15 to 94 (average, 52) years. There was a 2:1 male/female ratio. Fifty-five percent of the lesions occurred in the urinary bladder, 41% in the urethra, and 4% in the ureter. The most common microscopic pattern, present in 96% of the cases, was tubular. The tubules were usually small, hollow, and round, but some were solid and occasionally elongated. Their arrangement varied from orderly, sometimes laminar, to pseudoinfiltrative. A basement membrane was appreciable around the tubules in 25% of the cases but was rarely prominent. In two prostatic urethral cases tubules involved the fibromuscular prostatic stroma. A cystic pattern was seen in 72% of the cases. The tubules and cysts most commonly contained eosinophilic secretion but in 25% of the cases the secretion was basophilic. In 12% of the cases some of the tubules were tiny and when they contained basophilic secretion occasionally mimicked signet-ring cells. The third most frequent pattern, seen in 65% of the cases, was papillary to polypoid. Edematous polyps were commonest but thin, delicate filiform papillae were present in 10% of the cases. A complex branching pattern with prominent budding of small papillae was seen in three cases. The papillary-polypoid pattern was usually associated with a tubular component but in three cases it was pure. The final pattern, present in 14% of the cases, was diffuse, but it was prominent in only one case.(ABSTRACT TRUNCATED AT 250 WORDS)