Two-day-old specific pathogen-free (SPF) chickens were divided into two groups. Group I was inoculated orally with fowl adenovirus VIII (FAV-VIII). Group II served as a negative control. Chickens were investigated at various days post-inoculation (dpi) by flow cytometric analysis for changes in T lymphocyte subpopulations in immune system and blood. In the thymus, CD3+ T lymphocytes were increased at 25 dpi, with significant increases in the FAV infected noted at 1, 12, 20dpi (p<0.05). This was accompanied by a corresponding increase of CD4+ and CD8+ T lymphocytes. In the spleen, CD3+ and CD4+ T lymphocytes were increased significantly at 30 dpi (p<0.01) whereas CD8+ and TCR γ δ+ T lymphocytes were decreased at 1 (p<0.05), 30 dpi (p<0.01). An increase of CD3+, CD4+ and CD8+ T lymphocytes was noticed in peripheral blood, and accompanied by a decrease of TCR γ δ+ T lymphocytes. These results demonstrated that infection with FAV-VIII causes significant fluctuations in T lymphocyte subpopulations in thymus, blood and spleen. It can be concluded that an infection with FAV-VIII has profound effects on the immune system, especially on cell mediated immune competency.