Ten cases of luteinized unruptured follicle (LUF) syndrome out of 250 women with unexplained infertility were detected on ultrasonography, giving a frequency of 4%. Hormonal analysis revealed lower serum progesterone levels at mid-luteal phase in LUF cases, suggesting a link between LUF syndrome and inadequate luteal phase. Prolactin response to thyrotropin-releasing hormone was exaggerated in LUF cases as compared with ovulatory cases. Aberrant prolactin release may be a contributory factor in the pathophysiology of the LUF syndrome.