The possibility of prolactin-dependent subfertility was investigated in a group of 8 women, with luteal insufficiency exhibiting moderately elevated plasma prolactin (PRL) levels and/or galactorrhea. Another group of 10 normal women volunteers served as the control group. A "luteal index" was elaborated by integration of the area below the curve of plasma progesterone (P) values recorded throughout the postovulatory period. The calculated index for normal women was 177 +/- 35 (SD) expressed as [(ng/ml) x time], and the value of 107 (--2 SD) was adopted as the lower limit of normality (97.5% confidence limit). All 8 patients had luteal indexes (range 20--105) below the established limit. Therapy with bromocriptine (CB 154), 5 mg/day, suppressed PRL to normal levels and prolonged the postovulatory hyperthermic phase in 6 out of 8 women. This was accompanied by an improvement in the luteal index, and 5 women conceived. It is concluded that prolactin may interfere with normal progesterone synthesis by the corpus luteum, as demonstrated by the prompt restoration of fertility by bromocriptine treatment in women with regular cycles and inadequate luteal function.