Magnetic resonance imaging was used to detect signs of regeneration of the thymus after approximately one month of human growth hormone administration. A 46-year-old human volunteer was placed on a regimen of recombinant human growth hormone and pharmaceutical grade dehydroepiandrosterone for one month. Mediastinal magnetic resonance images were collected at baseline and after the study period. Thymic cross sections were analyzed for total area and for the total gray area, which was taken to represent functional mass. Baseline and post-treatment blood samples were taken to follow changes in IGF-1 levels and related metabolites. The setting was an informal, non-institutional trial supervised by a physician will full informed consent of the volunteer. Visual inspection and image analysis demonstrated limited but distinct enlargement of the thymus after treatment, and an increase in the percent of thymic cross section represented by gray-appearing (functional) mass. Estimated total thymic functional volume was within the normal range at baseline, but after treatment was more than three standard deviations above the expected mean for a subject of this age, thus meeting a proposed definition of thymic hyperplasia for individuals. IGF-1 levels were confined to the upper range of normal for young adults. The present observations apparently provide the first demonstration of growth hormone induced partial reversal of established thymic involution in a normal human subject, and are consistent with previous measurements of restored immune function after the administration of human growth hormone to elderly individuals.