To evaluate the significance of prolactin (PRL) disorders as one of the etiologic factors in habitual abortion, one hundred and six couples with a history of recurrent spontaneous abortion were assessed clinically and endocrinologically. A high rate of PRL disorders (eight patients with hyperprolactinemia and 31 patients with occulted hyperprolactinemia) was observed (36.8%) in habitual aborters, and only nine of these patients showed clinically detectable luteal insufficiency. These patients who had PRL disorders without luteal insufficiency were further analyzed with immunological examinations. Immunological evaluations revealed that there was no significant difference between the subpopulations of peripheral blood lymphocytes (PBL) in patients with PRL disorders and normal controls while the PHA-stimulated blastogenic activity of PBL and ConA-induced IL-2 synthesis by PBL was slightly suppressed as compared with that of normal controls. And the incidence of HLA sharing in couples with PRL disorders was demonstrated to be higher than that of the normal control couples without abortion. Patients with PRL disorders without impaired corpus luteum function were treated with bromocriptine only, and this therapy was very effective in maintaining pregnancy. The results of this study have allowed us to suggest that PRL disorders might be some of the etiologies in the so called "unexplained" habitual abortion.