In a total of 59 prostate cancer (PCa) patients, 9 patients with PIN. 29 subjects with BPH and 26 healthy men serum TPS and PSA values were measured together with NK cell activity, number and proportion of CD16+ cells, and reactivity of lymphocytes to mitogens (Con A. PHA and PWM). NK activity data indicate highly significant differences between both of patients with local tumor and those with disseminated disease (P < 0.01) and b) responders and nonresponding patients to hormonal therapy (P < 0.01). The number and proportion of CD16+ cells is lowest in BPH patients in comparison with controls and PCa patients. Since benign enlargement is attributed mainly to stromal cell proliferation in the absence of cell death in this compartment, gene expressions which control these events may participate in the surprisingly low CD16+ cell proportion. The reactivity of lymphocytes to mitogens (PHA. Con A and PWM) showed lower numerical values in all categories of PCa and BPH patients when compared with healthy men. The reactivity of T and B lymphocytes reported herein as immunological responses to mitogens (PHA. Con A and PWM) was performed 4-6 months after the beginning of therapy. Our data fit in well with those previously reported. Numerically lowest respective reactivity parameters to all mitogens were assessed in PIN subjects. Reported results show the specific significance of the changes in NK cell activity in regard with both metastatic extention of PCa and tumor response to therapy. These alterations match in their reliability changes with tumor marker values related to prostate cancer activity (TPS) and tumor differentiation (PSA). Lymphocyte reactivity to mitogens (Con A. PHA. PWM) may help in a subclinical discrimination between BPH and PIN patients that is still an important goal of clinical urology.