In order to evaluate the immune state of dogs suffering from pituitary-dependent hyperadrenocorticism (PDH), peripheral lymphocyte subsets were examined. Twenty seven PDH dogs and eight healthy control dogs were used in the current study. Eight healthy dogs served as the control group. Twenty seven PDH dogs were categorized into 4 groups based on their post serum cortisol concentrations by ACTH stimulation test: 2-5, excellent control (n = 8); 5-20, fair control (n = 7); >20, poor control (n = 4); and untreated (n = 8). Cell counts were executed with white blood cells (WBC), lymphocytes, CD3(+) (T lymphocytes), CD4(+) (Helper T lymphocytes), CD8(+) (Cytotoxic T lymphocytes), CD21(+) (B lymphocytes) cells in addition to calculating CD4(+)/CD8(+) ratio. Results indicated a significant difference in lymphocyte numbers and lymphocyte subset populations (CD3(+), CD4(+), CD8(+), and CD21(+) cells) between PDH and control dogs. Moreover, comparison of the PDH groups (excellent control; fair control; poor control; untreated) demonstrated that all groups had a significant decrease in lymphocytes numbers (CD3(+), CD4(+) and CD21(+) cell counts) as compared to control group. Meanwhile, no significant differences were observed in WBC counts and CD4(+)/CD8(+) ratio between groups. Furthermore, lymphocyte subset distribution in excellent control PDH dogs without concurrent disease (n = 4) better resembled that of control dogs as compared to PDH dogs with concurrent disease (n = 4). PDH dogs may be suffering from an immuno-depressed state as evidenced by significant differences in lymphocyte subset populations. Furthermore, treatment of both PDH and concurrent disease might improve lymphocyte subset distribution.