Although the opinion has long been held that undescended testicles are especially prone to undergo malignant change, neoplasms in this site are rare. As in all malignant neoplasms of the testicle, the prognosis is poor and the mortality rate high. Five cases of malignant tumor occurring in undescended testicles are here reported. In each instance quantitative bio-assay of the urinary prolan was carried out. Incidence and Etiology In an analysis of 452 testicular tumors Cunningham (1) found 40 in organs imperfectly descended, from which it would appear that new growths are over ten times as frequent in normally descended as in maldescended testes. In 42 operations for imperfectly descended testes, at the Bellevue Hospital, Keyes and MacKenzie (2) found no instance of malignant growth, and in 105 cases coming to operation at the Royal Victoria Hospital no malignancy was found (Wade, 3). Tanner (4) estimated the incidence of malignant tumors of the testicle, including both descended and undescended, as one in 2,000 male hospital admissions. In a series of over 100 cases of teratoma reported from the Veterans Facility, at Hines, Ill., but 3 were located in undescended testes (Owen and Cutler, 5). Dean (6 and 7), quoting Howard, who examined many thousands of men in London hospitals, reported that 0.06 per cent had undescended testicles. Our figure, calculated from over 40,000 male admissions is closer to 0.03 per cent. Dean has also stated that teratoma occurs about 237 times as frequently in undescended testicles as in those normally descended.