Luteal phase defects are defined as disorders resulting from abnormal corpus luteum function associated with insufficient progesterone production. The incidence is difficult to estimate accurately, but the disorder may affect 3-4% of infertile couples. Candidates for screening are those with unexplained infertility or recurrent abortion. Blood samples should be obtained seven to nine days after ovulation as determined by the thermogenic shift on basal body temperature monitoring or by a urinary luteinizing hormone surge. A midluteal phase serum progesterone level < 10 ng/mL is suggestive of the diagnosis. Endometrial biopsies are indicated in those couples with unexplained infertility and recurrent abortion, particularly if progesterone levels are > 10 ng/mL. While there have been few comparative studies, the four treatments available are clomiphene citrate, progesterone vaginal suppositories, human menopausal gonadotropins and bromocriptine. Because of its simplicity of use, clomiphene citrate is the recommended first-line treatment.