Patients with different types of luteal phase defects were studied with the use of the radioimmunoassay for the beta subunit of human chorionic gonadotropin (hCG) to determine if unsuspected subclinical pregnancies were more common in a particular type of defect. A type I luteal phase defect is always characterized by a chronologic lag in endometrial development when repeatedly studied with timed endometrial biopsies. A type II luteal phase defect is always characterized by an in phase endometrium when repeatedly studied by timed endometrial biopsies but always has less than a 14 day luteal span. All blood samples were drawn at least 7 days after ovulation/conception. In 22 cycles in which patients had a type I luteal phase defect, no subclinical pregnancies were detected. In 18 cycles in which a type II luteal phase defect was present, 12 instances of unsuspected subclinical pregnancy were detected and all ended in spontaneous abortion. This study shows that unsuspected subclinical pregnancies ending in abortion do occur and are quite commonly associated with the type II luteal phase defect.