Fifteen infertile women with inadequate luteal phase, histologically documented in at least two separate cycles, and normal midluteal plasma levels of progesterone (greater than or equal to 10 ng/mL), estradiol (70 to 300 pg/mL), and prolactin (less than 20 ng/mL) received "pure" follicle-stimulating hormone (pFSH), 150 IU intramuscularly, for 4 days (days 1 to 4 of the cycle). The endometrial defect was corrected in 7 of the 15 (46.7%) patients during the first treated cycle. Hormonal levels were similar in control and treatment cycles. Two of 5 patients with no additional infertility factors except luteal phase deficiency (LPD) became pregnant and carried to term singleton pregnancies. In 5 additional infertile patients with normal luteal function as assessed by endometrial histological study (2 cycles) and hormone measurements (first study cycle), a third biopsy was performed in a consecutive cycle under pFSH administration. In no case was the normal secretory pattern impaired. It is concluded that (1) some forms of LPD may be successfully treated by early follicular pFSH therapy and (2) pFSH does not alter the normal endometrial secretory pattern.