The observation of the "Peak" mucus symptom in women using the ovulation method of natural family planning has been correlated with the estimated time of ovulation, as evaluated by indirect hormonal parameters. In 65 cycles of the 73 studied in 24 patients, there was hormonal confirmation of ovulation; in eight cycles, anovulation or luteal dysfunction was suspected. In the 65 normal cycles, 64 exhibited a Peak symptom. In those cycles, ovulation was estimated to occur from 3 days before to 3 days after the Peak symptom with a mean of 0.31 days before the Peak symptom. In 95.4% of these cycles, ovulation was estimated to occur from 2 days before to 2 days after the Peak symptom. The variation between cycles of the same patient ranged from 0 to 4 days with a mean of 1.8 days. The beginning of the mucus symptom preceded the estimated time of ovulation by an average of 5.9 days.