Behavioral weight loss programs have been shown to be effective for short-term weight loss, however the impact of these programs on dietary changes is unclear. This study examined the changes in participant’s diet and physical activity over the course of a 16-week Internet behavioral weight-loss program. A single-center randomized controlled trial was conducted from August 2008 to December 2008 in Lexington, KY, and sixty-six women whose mean (SD) age was 48.6 (10.8) years and body mass index was 31.8 (3.7) kg/m2 completed all dietary and physical measures. Participants received two face-to-face group sessions with a dietitian, at baseline and 4-weeks, in addition to 16 weekly behavioral weight loss lessons delivered via an Internet website. Participants showed a significant reduction in energy intake (1879.2±771.7 vs. 1372.9±423.7; p<0.001), dietary energy density (2.1±0.5 vs. 1.9±0.5; p=0.002) and a significant increase in diet quality score as measured by the HEI-2005 (53.9±9.9 vs. 57.4±10.6; p =0.002). Participants did not show significant differences in physical activity intensity, duration or energy expenditure. However, post hoc analysis revealed that those who adopted a healthy life style, such as eating more fruits and vegetables and being physically active, achieved greater weight loss than those who did not adopt a healthy lifestyle. Participation in this Internet behavioral weight loss program significantly improved dietary intake in adult women and did not significantly improve daily physical activity levels.