Purpose: Immunohistochemical expression of CD3, CD4, and CD8 antigens in the thymus from birth to puberty was investigated in this study. We analyzed the histologic changes that occur in the thymus with age and the distrubution of cells that react with these antigens. Methods: Tissue samples were obtained from patients of Gazi University Faculty of Medicine, Department of Cardivascular Surgery. Age of the patients ranged from 11 months to 11 years old. Three of them were boys and five of them were girls. Both parraffin and cryo sections were taken and immunostaining was performed. Results: Thymocytes were seen in the cortex and medulla of all age groups. Definite apoptosis was not seen in the thymocytes. Immunohistochemical findings show that although strongly positive membranous and cytoplasmic staining was seen in the cortex thymocytes of early age groups, this staining seems to change from strongly positive to mild with age in thymus samples. The most positive reaction was seen with CD3 in the medulla of all age groups. CD8 immunoreactivity in the cortex was seen mostly in membranous staining. Conclusion: We observed that CD4 and CD8 had similar immunoreactions in the thymus. Reactive thymocytes include most of the cortex thymocytes and small amounts of medullar thymoctes. This reactivity was strongly membraneous and mild cytoplasmic. In addition, CD3 reactivity was similar in the cortex and medulla of all age groups. Findings were well-matched with the changing structure of the aging thymus. Major changes were not seen in different age groups. We estimated that the thymus involutes with age but thymocytes work with the same activity and efficiency.