Perfluorooctanoic acid (PFOA) has been used in commercial applications and detected in environmental matrices. This study focuses on whether PFOA affects the function of immune organs (spleen and thymus). Male ICR mice were exposed to 0, 2, 10, 50, and 250 ppm of PFOA in drinking water for 21 days. PFOA differently altered T lymphocyte populations. In the spleen, all doses of PFOA decreased CD8(+) lymphocytes; CD4(+) lymphocytes were increased by 50 and 250 ppm of PFOA. Exposure to 250 ppm of PFOA increased CD8(+) lymphocytes in the thymus. In the histopathological evaluation, the spleen of 250 ppm PFOA-treated groups revealed the increase of lymphoid hyperplasia of white pulp without significant alteration of red pulp. The thymus of 250 ppm PFOA-treated group showed decreased thickness of the cortex and medulla, but lymphoid cells were more densely arranged. PFOA elevated the expression of proinflammatory cytokines (tumor necrosis factor alpha, interleukin-1beta, and interleukin-6) in the spleen, and proto-oncogene, c-myc, in the spleen and thymus. In conclusion, our data demonstrated that PFOA has an immunomodulatory effect by altering T lymphocyte phenotypes and gene expression of proinflammatory cytokines.