To test the efficacy of the TwoDay Method, a new fertility awareness based method of family planning that provides women with simple instructions to identify the days each cycle when they are most likely to become pregnant. Users avoid unprotected intercourse on days when cervical secretions are present on that day or on the day before, to prevent pregnancy. Prospective, nonrandomized, multicenter study. Five culturally diverse sites in Guatemala, Peru, and the Philippines. Four hundred fifty women, aged 18- 39 years, wishing to use a fertility awareness based method to prevent or delay pregnancy. Study participants were followed for up to 13 cycles of method use. Life table pregnancy rate. The first year pregnancy rate was 3.5 (pregnancies per 100 women/years) with correct use of the method (pregnancies and cycles with no intercourse on identified fertile days), 6.3 with use of a backup method on the fertile days, and 13.7 including all cycles and all pregnancies in the analysis. The TwoDay Method offers a valuable addition to the services that reproductive health and other programs can offer. Its efficacy compares well with that of other coitusdependent family planning methods; it is easy to teach, learn, and use; and it can address the need of women for simple, accurate instructions for identifying their fertile days. To test the efficacy of the TwoDay Method, a new fertility awareness based method of family planning that provides women with simple instructions to identify the days each cycle when they are most likely to become pregnant. Users avoid unprotected intercourse on days when cervical secretions are present on that day or on the day before, to prevent pregnancy. Prospective, nonrandomized, multicenter study. Five culturally diverse sites in Guatemala, Peru, and the Philippines. Four hundred fifty women, aged 18-39 years, wishing to use a fertility awareness based method to prevent or delay pregnancy. Study participants were followed for method to prevent to delay pregnancy. The first year pregnancy rate was 3.5 (pregnancies per 100 women / years) with correct use of the method (pregnancies and cycles with no intercourse on identified fertile days, 6.3 with use of a backup method on the fertile days, and 13.7 including all cycles and all pregnancies in the analysis The TwoDay Method offers a valuable addition to the services that reproductive health and other programs can offer. Its efficacy than with the other coitus dependent family planning methods; it is easy to teach, learn, and use; and it can address the need of women for simple, accurate instructions for identifying their fertile days.