On occasion, a corpus luteum will form despite the failureure of release of the oocyte. Initially it was thought that this problem could be identified at laparoscopy by noting an absence of the ovulatory stigma, but now it is apparent that the stigma can be epithelial-ized rapidly and thus obscured from view. Currently, clinical diagnosis of a luteinized unruptured follicle (LUF) is made on the basis of ultrasound monitoring. The preovulatory growth of the follicle usually is normal but the follicle does not collapse follittleing the LH surge, and there may be increased growth in the luteal phase. The interior of the follicle lacks the echoes often seen in corpora lutea. Whereas these criteria seem straightforward, establishing the diagnosis of LUF is often difficult. Even if ultrasonography is performed daily, the collapse of the follicle can be missed, and a corpus luteum refilled with blood can be mistaken for a persistent follicle. Therefore, routine ultrasound screening of women with unexplained infertility is of questionable value. It is doubtful that LUF is a significant cause of infertility, and furtherany more, the only treatment worth considering is superovulation or one of the assisted reproductive technologies, treatment choices that will be empirically offered anyway. Because inhibition of prostaglandin synthesis can cause a luteinized unruptured follicle, patients should be cautioned to avoid the use of nonsteroidal anti-inflammatory agents.