Objective: To determine the effectiveness of an electronic hormonal fertility monitor plus cervical mucus monitoring to avoid pregnancy. Design: A 12-month prospective clinical efficacy trial. Setting and Participants: One hundred ninety five (195) women (mean age 29.8 years) seeking to avoid pregnancy with a natural method at 5 clinical sites in 4 cities. Intervention: Each participant was taught to track fertility by self-observation of cervical mucus and an electronic monitor that measures urinary levels of estrone-3-glucuronide and luteinizing hormone. Main Outcome Measures: Correct- and typical-use unintended pregnancy rates. Results: There were a total of 26 unintended pregnancies, 3 with correct use. With 1,795 months of use, the correct-use pregnancy rate was 2.1% per 12 months of use (i.e., 97.9% effective in avoiding pregnancy when rules of the method were always followed) and the imperfect-use pregnancy rate was 14.2% per 12 months of use (i.e., 85.8% effective in avoiding pregnancy when rules of the method were not always followed and all unintended pregnancies and months of use were included in the calculations). Conclusions: Correct use of an electronic hormonal fertility monitor with cervical mucus observations can be as effective as other fertility awareness–based methods of natural family planning. Comparative studies are needed to confirm this conclusion. JOGNN, 2007. DOI: 10.1111/J.1552-6909.2007.00129.x