The association between physical activity during childhood and breast cancer risk was examined. To study thisquestion data on physical activity in childhood were analyzed. A hospital-based case-control study of 250 Polishincident breast cancer cases (49.2% of eligible) and 301 (41.4% of all selected) frequency matched for age controlswas conducted in 2003-2004 in the Region of Western Pomerania. Women were asked to compare their total physicalactivity at ages 10-12 years and 13-15 years with the activity of their female peers by choose from one of threecategories: less active, equally active, more active, the best describing their activity. Odds ratios (ORs) and 95%confidence intervals (CIs) were calculated using univariate and multivariate logistic regression, fitted by the methodof maximum likehood. Women who reported having been physically more active than their peers at ages 10-12 yearshad an age-adjusted OR=0.88 (95% CI=0.36-2.15, P for trend=0.37) as compared with those reported being lessactive. Adjustment for potential confounders and total lifetime physical activity decreased the risk estimate to OR=0.25(95% CI=0.06-1.10, P for trend=0.15). For physical activity at ages 13-15 years, both an age-adjusted and multivariateadjusted ORs were also decreased among women who were at least such active as their peers, but the reductionswere not statistically significant. For women who were more physically active than their peers during both ageperiods the adjusted OR was 0.30 (95% CI=0.11-1.34, P for trend =0.21). These results show no protective role forphysical activity in childhood on breast cancer development among women aged 35-75 years. Further investigationsemploying larger sample sizes with comprehensive assessment of physical activity during the childish years areneeded to verify this evidence.