A prospective study is presented in order to determine the frequency of the luteinized unruptured follicle (LUF) in a population of 66 regularly cycling women. They were monitored by daily ultrasound for a total of 183 cycles, and the LUF was detected in 9 cycles, giving an incidence of 4.9%. The results of daily changes of luteinizing hormone, estradiol, and progesterone provide support for the thesis that the LUF behaves steroidogenically as a corpus luteum and that the luteal phase duration is normal. Continued monitoring in 35 cycles revealed a recurrence in only one case during a fourth subsequent cycle. Thus, the findings indicate that the LUF is a sporadic and infrequent phenomenon. It is therefore an uncommon cause of infertile cycles in potentially fertile women and represents a biologic variable rather than a syndrome. Based on ultrasonic and endocrine observations, a mechanism is proposed for the resolution of the LUF.