Serial ovarian ultrasound and daily assessments of plasma concentrations of pituitary and ovarian hormones were used to investigate ovarian function in 175 women with unexplained infertility. Their endocrine and ultrasound profiles were compared with similarly derived data from 43 normal volunteers. Fifty-one (29.1%) of the study group showed subnormal luteal phase rises in progesterone concentrations, described as poor progesterone surge (PPS) cycles. Within this group, 23 women (45.1%) demonstrated luteal cyst formation, a pattern not seen in any of the control cycles. High concentrations of follicle stimulating hormone (FSH) and reduced concentrations of oestradiol (E2) were observed in the follicular phases of the PPS cycles suggesting that the phenomenon is a product of abnormal follicular metabolism. An association of PPS with infertility exists, perhaps related to a combination of disturbances in the follicular micro-environment compromising oocyte quality, a failure of oocyte release, and impaired endometrial receptivity.