BACKGROUND: Physical activity interventions tailored to individual characteristics and delivered via print produce greater increases in activity compared with nontailored interventions and controls. Using the Internet to deliver a tailored physical activity intervention offers an alternative to print that might be available to larger populations at a lower cost. METHODS: Participants (N = 249 adults; mean [SD] age, 44.5 [9.3] years; and mean [SD] body mass index [calculated as weight in kilograms divided by height in meters squared], 29.4 [6.1]) were randomized to 1 of 3 physical activity interventions: (1) motivationally tailored Internet (tailored Internet, n = 81), (2) motivationally tailored print (tailored print, n = 86); and (3) 6 researcher-selected Web sites available to the public (standard Internet, n = 82). Participants in the tailored Internet and tailored print arms received the same tailored intervention content. Participants were assessed at baseline and at 6 and 12 months. RESULTS: At 6 months, participants in the tailored print arm reported a median of 112.5 minutes of physical activity per week, those in the tailored Internet arm reported 120.0 minutes, and those in the standard Internet arm reported 90.0 minutes (P = .15). At 12 months, the physical activity minutes per week were 90.0, 90.0, and 80.0 for those in the tailored print, tailored Internet, and standard Internet arms, respectively (P = .74). Results indicated no significant differences between the 3 arms. CONCLUSIONS: The use of tailored Internet, tailored print, and standard Internet as part of a behavior change program increased physical activity behavior similarly. Because the use of the Internet was not different from the print-based intervention, this may be an opportunity to reach more sedentary adults in a more cost-effective way. Trial Registration clinicaltrials.gov Identifier: NCT00200317.