This pilot study involved 20 cycles contributed by six apparently healthy women. They were all experienced users of the symptothermal method of NFP and trained as NFP-teachers. In a double-blind study NFP-parameters like S-19 = F1, appearance of any mucus = F2 and appearance of fertil mucus = F3 also peak mucus symptom +4 = L1 and 3. day of high basal body temperature = L2 are related to ovulation detected by ultrasonic measurement of follicular growth and hormonal values (LH, total Oestrogen in urine). We have focused on the calculation of the first day and the end of the fertile phase. In relation to the maximal follicular diameter (mfd = day 0) the LH-peak was located at day -0,7. The peak mucus symptom was observed at day -0,58. F1 was seen 10,4 +/- 2,6 days, F2 6,2 +/- 2,6 days, F3 2,8 +/- 1,4 days before day 0. L1 was located at 3,37 +/- 1,74 days, L2 4,1 +/- 1,95 days after day 0. Using at least 2 indicators as recommended with the symptothermal method none of 19 cycles failed to detect the beginning of the fertile phase, but 3 out of 20 failed to detect the end. In 19 out of 20 cycles natural signals observed by the participants indicated ovulation. It must be mentioned that all our women were highly motivated persons.