Short term overall survival is, regardless of treatment allocation, expected to be very close to 100 %. The primary outcome is relapse rate. The power of the study depends on the number of observed relapses. If the relapse rate in the adjuvant carboplatin group, the reference group, is lower than the anticipated 9 %, we need to include more patients to the study. Based on all previous published material on adjuvant treatment in clinical stage I seminoma it is not possible to precisely estimate the correct relapse rate until the median follow-up is four years. Consequently, we will estimate the relapse rate in the reference group close to the end of accrual. If the estimated relapse rate, and thus the number of relapses, is lower than the anticipated we will increase the sample size to make sure that the study meets the minimum required number of relapses in the reference group. A possible inclusion of more study participants does not compromise the Type I error rate of the study.