The symptoms of self-observation of the menstrual cycle (basal body temperature, mucus symptom, autopalpation of the cervix) are often regarded as not reliable for ovulation detection. In a prospective study 87 NFP cycles are monitored additionally with ultrasound and LH tests to calculate the correlation of the ovulation-time with the symptoms of self-observation. Our results show that the symptoms of self-observation allow a reliable detection of the time of ovulation. Only a short introduction into the method of self-observation is a necessary precondition. The reliable detection of ovulation gives the opportunity of cycle analysis of large groups especially in long time investigations. In this way a large set of valuable and reliable data on normal and disturbed menstrual cycles will be available.