In 30 normally menstruating women, ages 19 to 41 (mean 24), gravida 0 to 5 (mean 0.7), basal body temperature (BBT) was correlated with serum luteinizing hormone (LH), progesterone, and estradiol or urinary estrogen levels assayed serially during one menstrual cycle. In 21 subjects (70%), a biphasic BBT correlated with an ovulatory hormonal pattern. Six women (20%) had a monophasic BBT but demonstrated a preovulatory estrogen peak, a midcycle LH surge, and a significant rise in serum progesterone levels during the luteal phase. The remaining three women (10%) showed anovulatory cycles (two women) or a deficient luteal phase (one woman) as determined by BBT and hormonal assays. The results indicate that in approximately 20% of ovulatory cycles the BBT failed to demonstrate ovulation.