The nature and incidence of normal and abnormal spontaneous ovarian cycles, identified with ultrasound and endocrine tracking, were examined in 45 regularly cycling infertile women with no definitive cause and 15 women who were apparently normal and were receiving donor insemination because of clearly infertile partners. In 136 cycles, four apparently distinct abnormal patterns were detected. The total incidence in the infertile group was 58% compared with 23% in the donor insemination group (P less than 0.005). Twelve of 26 subjects who had at least three cycles tracked showed two different abnormalities, and 1 subject had three different abnormalities in five abnormal cycles. These results suggest that abnormal cycles are a significant factor in unexplained infertility and that diagnosis and treatment cannot be based on the study of a single cycle.