With cervical mucus, or Billings, method of family planning, a woman learns to recognize the characteristics of her cervical mucus that identify the fertile phase in her menstrual cycle. She and her partner abstain from sexual activity during a period from the first indication of mucus until four days after the mucus peak day, which includes ovulation. They also abstain during menses, because mucus can be confused with menstrual bleeding. This method of family planning is used widely, worldwide, by couples seeking a natural, reliable method of family planning. Between July 1988 and May 1990, 688 couples of child-bearing age, most of whom were parous, used the Billings method for contraception. Five hundred and fifty of these couples used the method for more than 12 months. Efficacy, continuation rates, and discontinuation rates were analyzed using life-table analysis for 10,175 woman-months of data collected. The net cumulative discontinuation rates per 100 women at 12 and 18 months were 19.85 and 34.58, respectively, resulting in continuation rates of 80.15 and 65.42. The discontinuation rates per 100 women for method-related reasons at 12 and 18 months were 1.61 and 2.84, respectively, while the discontinuation rates for unintended pregnancy were 1.02 and 1.18. During the study, 67 subjects volunteered to have vaginal smears taken from the upper part of the vaginal wall for cytologic examinations, and among them serum and urine LH levels were measured in 10 subjects and urinary estrogen and progesterone were assayed in 35 cases. These tests related other indications of the menstrual cycle to the ability of the women to judge ovulation by cervical mucus. Our research suggests that further investigation of the Billings method is warranted and that careful planning and organization are needed to disseminate the method more broadly.