Recent investigations suggested a possible immunologic etiology for endometriosis. To determine whether cellular immunity may be affected in patients with endometriosis, peripheral lymphocyte populations were investigated in 31 confirmed endometriosis patients and 22 control patients. No statistical differences in OKT3, OKT4, OKT8, OKM1, and OKTa1 cells could be observed. Total lymphocyte numbers were within normal ranges for both groups. Helper/suppressor ratios were not statistically different and were in a normal range for both groups. It is concluded that numeric differences in lymphocyte subpopulations represent only a very unlikely contributing factor in a potential immunologic etiology of endometriosis.