The incidence of second primary cancer (SPC) was determined in 64 irradiated stage I testicular seminoma patients, treated and followed at the Northern Israel Oncology Center from 1968 to 1988. Seven (11%) patients developed a total of eight second cancers. The cumulative risk for developing second primary cancer at ten, 15 and 20 years following the diagnosis of seminoma was 2.5%, 5.1% and 8.9%, respectively. Two patients developed SPC within the radiation field (urinary bladder, sigmoid colon) and four patients outside the radiation field (bronchogenic cancer, thymoma, malignant melanoma, thyroid cancer). In two patients, who developed lung cancer and testicular seminoma, respectively, scatter dose from the main radiation field could not be excluded as one of the factors contributing to the SPC. Three patients died as a direct result of their SPC. It is concluded that there is a low, but significant risk of solid SPC among patients apparently cured of early stage seminoma. It will be important in future surveillance studies of stage I seminoma to assess the risk of SPC in the non-irradiated patients.