Purpose: To determine the effectiveness of the Marquette Method (MM) of natural family planning (NFP) as a method of avoiding pregnancy. Study Design and Methods: This was a 12-month retrospective evaluation of the MM system of NFP. Two hundred and four women (mean age, 28.6 years) and their male partners (mean age, 30.3 years) who sought to learn a method for avoiding pregnancy with the MM from four clinical sites were taught to track their fertility by self-observation of cervical mucus, by use of an electronic monitor that measures urinary levels of estrone-3-glucuronide and luteinizing hormone, and by use of basal body temperature. All unintended pregnancies were evaluated by professional nurses as to whether they were intended or not. Pregnancy rates over 12 months of use were determined by survival analysis. Results: There were a total of 12 unintended pregnancies, only 1 with correct use. The 12-month “correct use” pregnancy rate was 0.6 (i.e., 99.4% effective) and the “typical use” (total pregnancy rate) was 10.6 (i.e., 89.4% effective) per 100 users. Clinical Implications: When used correctly, the MM system of NFP is an effective means of avoiding pregnancy. The efficacy of the MM system includes proper preparation of the professional nurse NFP teachers.