A 2-year international study involving NFP (natural family planning) associations in 5 countries (Canada Colombia France Mauritius U.S.) was conducted to evaluate statistically the effectiveness of the sympto-thermal method an NFP method which adds to the temperatiure method the observation of signs and symptoms of the ovulatory period. 1022 couples (21736 couples) were selected on the following criteria: 1) the wife had to be between 19 and 44 years of age 2) the wife had to be of proven fertility by having carried a fetus for at least 28 weeks 3) the couple must have submitted 1 satisfactory temperature graphy before being accepted and 4) the couple must have expressed a willingness to submit temperature graphs for 2 years. 826 couples completed 24 months. Menstrual cycle length varied from 9 to 98 days with an average of 28.43 days. 205 women had a variation of 8 days or less. 128 unplanned pregnancies occured giving a rate of 7.47 conceptions/100 woman-years of exposure. Theoretical effectiveness was .93 pregnancies/100 woman-years using the Pearl Formula; 16 pregnancies occured in couples following instructions. Failure rate was 4.13% for couples trying to prevent pregnancies and 14.83% for couples only delaying a pregnancy. Failure rate of the method used alone was 7.16%; 8.19% used with other contraceptives. There was an average of 13 cycles per year per woman. Using the modified Pearl index (pregnancies per 1300 cycles rather than per 1200 months) the pregnancy rate was 7.66% vs. 7.47% for 1200 months. The study shows that the sympto-thermal method is effective and reliable. 2 issues remain unresolved: 1) whether a large group of people will accept a method that depends on periodic abstinence or 2) whether the method will be just as effective when used by couples who are not as well motivated as the participants in this study.