The luteinized unruptured follicle syndrome is a form of anovulation and a subtle cause of female infertility. The syndrome cannot be diagnosed by traditional progesterone-dependent ovulation detection methods. Without the use of invasive procedures or sophisticated equipment, the luteinized unruptured follicle syndrome may go unnoticed. The patient diagnosed as ovulatory, on the basis of traditional ovulation detection methods, who does not conceive may be experiencing the luteinized unruptured follicle syndrome, and thus infertility. The syndrome's incidence, detection, etiology, and treatment are described.