A prospective study of the reliability of the CUE Fertility Monitor to identify the fertile time of the menstrual cycle was conducted. The device provides a digital measurement of the electrical resistance of saliva (SER) and vaginal secretions (VL). The readings of the device were compared with the fertile time detected by ultrasonography of the growing follicle and estimation of the LH surge to detect ovulation. Thirteen women participated in the study. Sixteen cycles contained both signals (SER, VL) for the beginning and the end of the fertile period. In 2 cycles a CUE signal could not be found by the device. Using a computerized algorithm for evaluation of the CUE signals, the beginning of the fertile period was accurately detected in 14 cycles; in 2 cycles the signal was found less than five days prior to ovulation. The last day of fertility was identified correctly by the CUE Fertility Monitor in 10 cycles; in 6 it was incorrectly identified during the time when the woman was still fertile. This suggests that the CUE Fertility Monitor utilizing the algorithm on which it currently is based cannot be recommended for natural family planning. However, we think that the algorithm for evaluation should be improved as useful signals in the menstrual cycle are detected.