The thickness of the largest thymic lobe at computed tomography (CT) was measured retrospectively in 21 relapse free patients who had undergone chemotherapy for non-seminomatous testicular cancer. CT was performed at initial staging, at completion of chemotherapy and 3 to 12 months later. Enlargement of the thymus occurred in 7 patients, one of whom had a reduced thymic size at the first two examinations after chemotherapy. A temporary reduction was detected in another patient in whom the original thymic size was regained 19 months after start of chemotherapy. The mean age of the group with thymic enlargement was 21 years compared with a mean age of 28 years in the group with no increased size. The age difference between the two groups was significant (p less than 0.05, Mann-Whitney U test). To reduce the number of explorative thoracotomies the frequent occurrence of thymic enlargement after chemotherapy for non-seminomatous testicular cancer should be kept in mind.