We reviewed the clinical characteristics, treatment methods, and outcome of 26 patients presenting with seminoma arising in an undescended testis. The tumor-bearing testis was located in the iliac fossa in 17 patients and in the inguinal canal in nine. The most common presenting symptoms were pain or an enlarging mass. At diagnosis, the tumors tended to be relatively advanced, and 16 of 26 patients (62%) had nodal metastases. In addition to the paraaortic nodes, metastases were frequent to the ipsilateral iliac and inguinal nodes even in the absence of prior inguinal-scrotal surgery. Four patients had documented pelvic peritoneal tumor seeding. All patients received treatment in addition to resection of the primary tumor. Fifteen patients received radiotherapy only, 10 received chemotherapy only, and one received both modalities. At a median follow-up of 9.6 years, only one patient had relapsed. He was initially treated with radiation and after relapse was successfully salvaged with chemotherapy. The 5 and 10-year disease-free rate was 96%, and the 5 and 10-year survival rates were 92% and 79%, respectively. Appropriate treatment of seminoma arising in an undescended testis results in an excellent outcome equivalent to that observed for the more usual scrotal seminomas.