This study was undertaken to evaluate the parameters used for monitoring ovulation in patients on clomiphene citrate treatment for the induction of ovulation. The mean serum oestradiol level for every follicle larger than 17 mm on the day before ovulation was 1 885 pmol/l. This value is significantly higher than the value found in spontaneous cycles. The maximum average pre-ovulatory follicle size was 22 mm, also significantly larger than in spontaneous cycles. The maximum pre-ovulatory cervical mucus score is significantly lower than that found in spontaneous cycles. The average midluteal serum progesterone and oestradiol values were significantly higher than in spontaneous cycles. These differences should be taken into consideration in interpreting the results of patients treated with clomiphene citrate for ovulation abnormalities and before in vitro fertilization. Seven patients with a healthy cervix had poor cervical mucus scores in spite of high pre-ovulatory serum oestradiol levels; scores did not improve with administration of exogenous oestrogen, but became optimal with subsequent human menopausal gonadotrophin treatment. Poor cervical mucus despite ovulation and adequate pre-ovulatory serum oestradiol levels in patients on clomiphene treatment should therefore be considered as an indication for gonadotrophin treatment. Pre-ovulatory administration of human chorionic gonadotrophin had no effect in increasing midluteal serum progesterone and oestradiol levels.