Photodynamic therapy (PDT) is a promising treatment modality for malignant tumors but it is also immunosuppressive which may reduce its therapeutic efficacy. The purpose of our study was to elucidate the role of CD4+ and CD8+ T cells in PDT immunosuppression. Using silicon phthalocyanine 4 (Pc4) as photosensitizer, nontumor-bearing CD4 knockout (CD4−/−) mice and their wild type (WT) counterparts were subjected to Pc4-PDT in a manner identical to that used for tumor regression (1 cm spot size, 0.5 mg kg−1 Pc4, 110 J cm−2 light) to assess the effect of Pc4-PDT on cell-mediated immunity. There was a decrease in immunosuppression in CD4−/− mice compared with WT mice. We next examined the role of CD8+ T cells in Pc4–PDT-induced immunosuppression using CD8−/− mice following the same treatment regimen used for CD4−/− mice. Similar to CD4−/− mice, CD8−/− mice exhibited less immunosuppression than WT mice. Pc4–PDT-induced immunosuppression could be adoptively transferred with spleen cells from Pc4–PDT treated donor mice to syngenic naive recipients (P < 0.05) and was mediated primarily by T cells, although macrophages were also found to play a role. Procedures that limit PDT-induced immunosuppression but do not affect PDT-induced regression of tumors may prove superior to PDT alone in promoting long-term antitumor responses.