PURPOSE: The objective of this study is to examine the role of early lifetime exposure to physical activity on magnetic resonance imaging-determined breast density measures. METHODS: Associations of adolescent (high school (ages 14-17 yr) and early adulthood, post-high school (ages 18-21 yr) and past year) leisure-time physical activity, as well as a principal component score including all three estimates, were examined with percent dense breast volume (%DBV) and absolute dense breast volume (ADBV) in a cross-sectional analysis of 182 healthy women, ages 25-29 yr, enrolled in the Dietary Intervention Study in Children Follow-up Study (DISC06). Generalized linear mixed models were used to examine associations after adjustment for relevant covariates for the entire analytic sample. Analyses were repeated in nulliparous women and hormonal contraceptive nonusers. RESULTS: Physical activity during high school and post-high school were not statistically significantly related to %DBV or ADBV in multivariable models. Past year physical activity was positively related to %DBV in the unadjusted and partially adjusted models (P < 0.001 and P = 0.01, respectively), which did not adjust for body mass index (BMI). After additional adjustment for childhood and early adulthood BMI, this association became nonstatistically significant. The relation between past year physical activity and ADBV was not statistically significant. These findings were similar in nonusers of hormonal contraceptives. No statistically significant relations were found in nulliparous women or between the principal component score and %DBV or ADBV. CONCLUSION: Results from this study are consistent with previous research suggesting that physical activity during adolescence and early adulthood is unrelated to breast density. Copyright © 2013 by the American College of Sports Medicine.