Epidemiologic studies suggest that physical activity reduces breast cancer risk by 20-40 %. However, prior studies have relied on measures of self-report. In a population-based case-control study, we evaluated accelerometer measures of active and sedentary behavior in relation to breast cancer among 996 incident cases and 1,164 controls, residents of Warsaw, Poland (2000-2003), who were asked to wear an accelerometer for 7 days. Accelerometer values were averaged across valid wear days and summarized as overall activity (counts [ct]/min/day); in minutes spent in sedentary behavior (0-99 ct/min); and light (100-759 ct/min) and moderate-to-vigorous (760+ ct/min) activity. Odds ratios (OR) and 95 % confidence intervals (CI) were estimated using unconditional logistic regression. Comparing women in the highest quartile (Q4) of activity to those in the lowest (Q1), time spent in moderate-to-vigorous activity was inversely associated with breast cancer odds after adjustment for known risk factors, sedentary behavior and wear time (OR(Q4vsQ1) 0.39, 95 % CI 0.27-0.56; P-trend < .0001). Sedentary time was positively associated with breast cancer, independent of moderate-to-vigorous activity (OR(Q4vsQ1) 1.81, 95 % CI 1.26-2.60; P-trend = 0.001). Light activity was not associated with breast cancer in multivariable models including both moderate-to-vigorous activity and sedentary behavior. Our findings support an inverse association between accelerometer-based measures of moderate-to-vigorous physical activity and breast cancer while also suggesting potential increases in risk with sedentary time.