The aim of this study was to determine the influence of elevated serum prolactin (PRL) levels on the peripheral lymphocyte subsets in patients with hyperprolartinemia. For this purpose we studied 20 hyperprolactinemic patient lymphocyte subsets by flow cytometry on their hyperprolactinemic state and after their serum prolactin concentration was normalized with bromocriptine (BC) alone or BC and surgery. We observed decreased absolute numbers and percentage of Natural Killer (p=0.0009 and 0.0001, respectively) and CD3/CD25 lymphocytes (p= 0.009 and 0.002) in hyperprolactinemic patients, compared to 8 sex- and age-matched normal controls. There was no correlation between PRL levels and CD16/56 and CD3/CD25 numbers (p=0.72 and 0.33, respectively). We did not find any significant difference in absolute numbers (p=0.95) and percentage (p=0.84) of B-lymphocytes of hyperprolactinemic patients, as compared with normal controls. We did not detect any increase in absolute cell numbers of CD16/CD56 (p=0.21) and CD3/CD25 (p=0.61) of BC-treated patients when compared to their hyperprolactinemic state. We demonstrated an increase in CD8-cells (p=0.0173) and a decrease in CD4/CD8 ratio (p=0.036) in hyperprolactinemic patients treated with BC. There was also an increase in the number of activated T-cells (CD3/HLA DR) in this group, compared to normal controls and the hyperprolactinemic state of the same patients (p=0.04). In conclusion, elevated PRL levels do not lead to an "overstimulation" of the B-cells, but deteriorate the cytotoxic function.