To evaluate the role of ultrasound in diagnosing luteinized unruptured follicle (LUF), 37 women with unexplained infertility were examined for two to three menstrual cycles. Laparoscopy or laparotomy was performed on days 16 to 18 of the third study cycle in 25 patients. The LUF syndrome was suspected at ultrasound examination in 57 of 100 cycles observed. In the remaining 43 cycles, follicular collapse was observed in 33, and 10 were diagnosed as anovulatory. At laparoscopy or laparotomy on 25 patients, 18 of the 21 patients diagnosed as having LUF by ultrasound had a corpus luteum without a stigma. The other three cases diagnosed as LUF by ultrasound had ovulation stigmata. Additional findings in the 25 patients who underwent laparotomy or laparoscopy were endometriosis in 7 (5 of whom had LUF as well), ovulation in 5, bilateral hydrosalpinx in 1, and inability to visualize the ovaries because of adhesions in 1. The LUF syndrome was not a consistent change in the ovulatory pattern of most of the patients. It occurred by ultrasound diagnosis in three consecutive cycles in only 34% of patients.