Since the result of radiation treatment of cancer is constantly improving, special attention is now being focused on the dose received by the gonads during radiation therapy, especially those in younger age groups. The author analyse 10 cases of lung cancer teated with Co-60 teletherapy to the mediastinum and 10 cases of rectal cancer to the pelvis by using lithium fluoride thermoluminescent dosimeters (TLD) to measure testicular dose at department of therapeutic radiology of SNUH from August to September 1979. The results are as follows; 1. The testicular dose was 0.146% of the tumor dose in cases of mediastinal irradiation. 2. The testicular dose was 22.32% of the tumor dose in cases of pelvic irradiation. Still there is no evidence of detectable gane mutation increases linearly with increasing dose, and moreover no evidence for a threshold dose. In order to avoid or reduce permanent sterility or possible undersirable genetic effect, testicular dose should be minimized durin radiation the therapy.