Tamoxifen at a dose of 10 mg/day for 5 days was given to five infertile women in the luteal phase. Daily serum samples were obtained during the luteal phase for radioimmunoassay of progesterone (P), estradiol (E2), follicle-stimulating hormone, luteinizing hormone (LH), and prolactin levels. The integrated luteal phase concentrations of serum P and E2 before and after cycles of tamoxifen treatment increased from 87.8 +/- 16.2 ng/ml and 1120 +/- 164.4 pg/ml to 131.6 +/- 18.9 ng/ml and 1461 +/- 205.2 pg/ml, respectively (P less than 0.01 and P less than 0.05). No apparent increase in circulating LH levels was seen in one of the five cases, but this patient's serum P and E2 levels rose nonetheless. This suggests that the significant increase in circulating P and E2 induced by tamoxifen is not consistently associated with an increase in serum LH concentration.