Time of ovulation as detected by a self-test of luteinizing hormone (LH) in the urine was compared with time of ovulation as detected by self-observation of cervical mucus. Twenty regularly cycling women monitored their cervical mucus and urine LH for two complete menstrual cycles. Of the cycles that had an LH surge, 100% were on the peak day of cervical mucus or within three days before the peak day. Self-observation of cervical mucus, therefore, can be an accurate method of determining optimal fertility.