Immunohistochemistry was used to investigate the leukocyte populations in the endometrium of women suffering recurrent miscarriage. Mid-luteal phase endometrial biopsies were taken from 22 patients with idiopathic recurrent miscarriage and from nine women with normal obstetric histories. The samples were dated histologically and stained with a panel of monoclonal antibodies to identify leukocytes. The outcome of any pregnancy in subsequent cycles following the biopsy was determined. Similar numbers of cluster designation (CD)3(+) and CD8(+) cells were seen in both groups. However, CD4(+), CD14(+), CD16(+), CD56(+) and MHC class II(+) cells were significantly higher in the recurrent miscarriage group than in the controls. Two patients had B cells (CD22(+)) in their endometrium. No CD57(+) cells were seen in the controls; however, eight of the patients had a few CD57(+) cells present. Only two patients, both from the recurrent miscarriage group, had CD69(+) leukocytes in their endometrium. Patients who had miscarriages following endometrial biopsy had significantly more CD4(+), CD8(+), CD14(+), CD16(+), and CD56(+) leukocytes in their endometrium than either those who had live births or women with proven fertility. A different population of leukocytes was found in the pre-implantation endometrium from recurrent miscarriage patients as compared to those from fertile controls. These differences were accentuated in women who had a miscarriage subsequent to the biopsy compared with those who subsequently had a live birth.