PURPOSE: To determine the influence of mutual motivation on unintended pregnancy rates of couples who used natural family planning (NFP) methods to avoid pregnancy. STUDY DESIGN AND METHODS: Using an online taught NFP method, 358 women and (their male partners) indicated "how much" and "how hard" they wished to avoid pregnancy on a scale of 0 to 10 before each menstrual cycle charted over 12 month of use. This motivation scale is used in the National Survey of Family Growth as a measure of motivation. All pregnancies were verified with an online pregnancy evaluation and urine-based pregnancy test. A combined motivation score was used in analysis. RESULTS: There were 28 pregnancies among the low-motivation participants (N = 60) and 16 among the high-motivation participants (N = 298). At 12 months of use, there were 75 pregnancies per 100 users for the low-motivation group and only 8 for the high-motivation group. There was an 80% greater likelihood of a pregnancy with the low-motivation group (χ = 25.5, p