The incidence of second primary cancers was investigated in 6187 Danish men diagnosed with testicular cancer in the period 1943-1987. During the course of 59,000 person years, 459 subsequent primary cancers occurred. The relative risks were significantly increased for leukaemia, gastric cancer, pancreatic cancer, bladder cancer, non-melanoma skin cancer and kidney cancer. Increased incidence was furthermore suggested for cancer of the rectum, prostate and lung. The increased incidence of leukaemia appeared in the first 10 years after testicular cancer diagnosis. The excess incidence for gastric cancer, pancreatic cancer, rectal cancer and lung cancer was strongest 10-19 years after testicular cancer, while the relative risk of non-melanoma skin cancer and prostate cancer increased throughout the period of follow-up. The increased incidence of cancer in this cohort is most likely an effect of radiotherapy used for testicular cancer. It is proposed that the different incidence patterns over time after testicular cancer diagnosis reflect differences in the growth rate of tumours originating in different tissues.