Sixty Sertoli cell tumors of the testis, excluding large cell calcifying and sclerosing subtypes, are described. Patient age ranged from 15 to 80 years (mean, 45 years). The initial manifestation was usually a testicular mass; in 14 cases it had been enlarging slowly for a period of up to 14 years (mean 3.7 years). Only five patients had testicular pain. Four patients had metastatic disease at the time of presentation. All the tumors were unilateral and ranged from 0.3 cm to 15 cm (mean 3.6 cm). They were typically well circumscribed. Sectioning usually disclosed firm, tan-gray, white, or yellow tissue with areas of hemorrhage and a minor cystic component in approximately one third. Microscopic evaluation usually revealed diffuse sheets or large, nodular aggregates of tumor cells, within which solid or hollow, sometimes dilated, tubules and, less often, cords were usually at least focally identifiable. A relatively acellular, often vascular, fibrous to hyalinized stroma was frequently conspicuous. The tumor cells typically had moderate amounts of pale to lightly eosinophilic cytoplasm, but 10 tumors had cells with abundant eosinophilic cytoplasm. Large cytoplasmic vacuoles were prominent in 26 tumors. Nuclear atypicality was absent or mild in 54 cases, moderate in 4 cases, and marked in 2 cases. Mitotic rate ranged from less than 1 to 21 per 10 high power fields, with 50 tumors having no or only rare mitoses. Vascular space invasion was present in 11 cases and was prominent in 8. Follow-up of more than five years (average 8.4 years), or until evidence of metastasis was seen, was available for 16 patients. Nine were alive and well with no evidence of disease. Four were alive with disease and three died of disease. The pathologic features that best correlated with a clinically malignant course were as follows: a tumor diameter of 5.0 cm or greater, necrosis, moderate to severe nuclear atypia, vascular invasion and a mitotic rate of more than 5 mitoses per 10 high power fields. Only one of nine benign tumors for which follow-up data of 5 years or more were available had more than one of these features, whereas five of seven malignant tumors had at least three.