A large prospective long-term study with users of natural family planning (NFP) methods has been conducted to analyze the relation between unintended pregnancy rates and sexual behavior with special reference to barrier method use in the fertile phase. Seven hundred and fifty eight NFP beginners, 19-45 years of age, 14870 cycles, 28 unintended pregnancies were studied. Of the couples, 54.2% use NFP only or predominantly and 45.9% use mixed methods (additional barrier method use in the fertile phase in 55.7% of the cycles). The overall pregnancy rate after 12 cycles of exposure is 2.2% according to the actuarial method. There is no significant difference between NFP users and mixed methods users and also no significant effect of duration of use in the first 5 years of exposure. During "perfect use" the pregnancy rate at 12 months is 0.63%. When only protected intercourse takes place in the fertile phase the pregnancy rate is 0.45%. The symptothermal method of NFP is most unforgiving for imperfect use (unprotected intercourse in the fertile phase). However, it is extremely effective when either abstinence or protected intercourse is used in the fertile phase.