Background: In the Women’s Health Initiative (WHI) Dietary Modification (DM) randomized trial, after 8.5 years dietary intervention and 19.5 years cumulative (median) follow-up, dietary intervention participation was associated with a statistically significantly 22% lower breast cancer mortality (P = 0.02). In observational studies, physical activity has been associated with lower breast cancer risk with emerging results now indicating, compared to inactivity, any increase in physical activity has health benefits. Currently, longitudinal data on whether an intervention targeting dietary change influences other health-related behaviors as a gateway effect is limited. To evaluate whether randomization to a dietary intervention was associated with self-directed change in physical activity. Methods: In the WHI DM trial, 48,835 postmenopausal women, ages 50-79 years, with no prior breast cancer and baseline normal mammogram were randomized at 40 US clinical centers to a dietary intervention (19,541) or a comparison group. Dietary goals were to reduce fat intake to 20% of energy and increase intake of vegetable, fruit, and grains addressed in 18 group sessions in year 1 then quarterly. Neither randomization group received specific or ongoing instructions to increase physical activity, but physical activity was referenced in written materials given to the intervention groups in 7 of the 56 sessions. Episodes per week of moderate or vigorous recreational physical activity (MVPA) were collected at baseline and serially through 15.9 years follow-up by self-report questionnaire. Marginal longitudinal logistic regression models were used to assess physically inactive (MVPA = 0) or physically active (MVPA > 0) participants by randomization group. Marginal Poisson regression models estimated mean weekly MVPA by randomization group. Results: 45.6% of participants reported 0 MVPA at baseline which largely persisted throughout follow-up. During cumulative follow-up, relative to the comparison group, dietary intervention group participation was associated with 7% lower physical inactivity rate (odds ratio [OR] 0.93 95% confidence interval [CI] 0.91, 0.95, P < 0.001) and a 4% higher mean MVPA (ratio of means [RM] 1.04 95% CI 1.02, 1.06, P < 0.001). The association between dietary intervention participation with higher physical activity level was stronger with increasing BMI (P-interaction 0.01) and for women with waist circumference ≥ 88 cm (P-interaction 0.02). Conclusions: In conclusion, in a randomized trial setting, a low-fat dietary pattern intervention was associated with a significantly lower physical inactivity rate and significantly higher moderate and vigorous physical activity level which could be associated with health benefits. Clinical trial information: NCT00000611.