The aim of this study was to evaluate the risk of developing a second germ cell cancer (SGCC) among male patients with a primary germ cell cancer (PGCC). SGCCs, all metachronous, developed in 60 out of 2201 males treated for a PGCC at the Norwegian Radium Hospital in Oslo from 1953 to 1990. Further, 8 patients had synchronous germ cell cancers. The relative risk (RR) of developing an SGCC was 27.6 (95% confidence interval (CI) 21.1-35.6), and the cumulative risk at 15 years of follow-up 3.9% (95% CI 2.8-5.0%). In patients with primary non-seminoma, the cumulative risk of an SGCC at 15 years of follow-up was 5.0% and in patients with primary seminoma 3.4%. Patients less than 30 years of age had a higher cumulative risk of 7.8% compared to 2.1% in older patients at 15 years of follow-up. The RR of an SGCC, however, was equal in patients with primary seminoma and in patients with primary non-seminoma. If the interval between PGCC and SGCC was < 5 years, the PGCC was most often followed by an SGCC of same histological type. Treatment applied for the PGCC did not seem to be of significant importance for the development of SGCC. In conclusion, patients with PGCC have high RR of developing an SGCC and age group < 30 years display an especially high cumulative risk.