OBJECTIVE: To evaluate the efficacy, knowledge of fertility, and acceptability of a web-based natural family planning (NFP) education and service program. DESIGN: A 6-month repeated measure longitudinal evaluation pilot study. SETTING: A university based online website. PARTICIPANTS: The website was piloted with 468 volunteer women seeking NFP services. Of these participants, 222 used the automatic online fertility charting system to avoid pregnancy. The 222 charting participants had a mean age of 29.9 years (SD=5.6), 2.2 children (SD=1.9), 37% were postpartum, and 47% had regular menstrual cycle lengths. INTERVENTION: Nurse-managed web-based NFP education and service program. OUTCOMES: Pregnancies were confirmed by an online self-assessed pregnancy evaluation form. A 10-item fertility quiz and 10-item acceptability survey was administered online. RESULTS: Among the 222 users avoiding pregnancy, at 6 months of use, there were two correct-use unintended pregnancies that provided a pregnancy rate of 2% and seven total unintended pregnancies providing a typical use pregnancy rate of 7%. Mean knowledge of fertility increased significantly from time of registration (8.96, SD=1.10) to 1 month of use (9.46, SD=.10), t=4.60, p