To determine the incidence of both isolated and repetitive prolactin elevations in the luteal phase of otherwise normoprolactinemic women. To see if sporadic luteal-phase hyperprolactinemia is associated with progesterone deficiency, and to explore a possible physiological basis for sporadic hyperprolactinemia by TRH challenge.Hospital-based reproductive endocrinology/infertility service.Prospective measurement of luteal phase serum progesterone and prolactin in normoprolactinemic ovulatory women. TRH stimulation testing in volunteers with repetitive luteal phase hyperprolactinemia and normoprolactinemic controls.133 sequentially selected infertile, ovulatory women with normal prolactin levels in the proliferative phase of the cycle.Measurement of serum progesterone and prolactin during the luteal phase, based on the day of the LH surge. TRH testing in the midluteal phase of the cycle in patients with two or more luteal phase prolactin elevations, and in five normoprolactinemic volunteers in both the preovulatory and midluteal phase.Of 133 subjects, 85 (64%) had no prolactin level exceeding 20 ng/mL in the luteal phase. Thirty-three (25%) had two or more elevated levels, and were considered to have repetitive luteal phase hyperprolactinemia (LPH). TRH testing in control subjects resulted in a greater prolactin response in the preovulatory phase. The group with LPH demonstrated an initial elevation of prolactin greater than that of the normoprolactinemic controls, but a subsequent drop to levels lower than both preovulatory and midluteal normoprolactinemic controls by 45 minutes.Sporadic luteal-phase hyperprolactinemia is a relatively common event (36% of 133 subjects in the present series). Of these 48 women, 33 (69%) had repetitive elevations, suggesting the elevation in these subjects to be more than a random event. The physiological validity of this observation is further demonstrated by an abnormal response to TRH stimulation, but the normal levels of luteal phase progesterone leave questions as to its pathological importance.