BACKGROUND: The aim was to compare the efficacy and acceptability of two Internet-supported fertility-awareness-based methods of family planning. STUDY DESIGN: Six hundred and sixty-seven women and their male partners were randomized into either an electronic hormonal fertility monitor (EHFM) group or a cervical mucus monitoring (CMM) group. Both groups utilized a Web site with instructions, charts and support. Acceptability was assessed online at 1, 3 and 6 months. Pregnancy rates were determined by survival analysis. RESULTS: The EHFM participants (N=197) had a total pregnancy rate of 7 per 100 users over 12 months of use compared with 18.5 for the CMM group (N=164). The log rank survival test showed a significant difference (p<.01) in survival functions. Mean acceptability for both groups increased significantly over time (p