The association between physical activity during childhood and breast cancer risk was examined. To study this question data on physical activity in childhood were analyzed. A hospital-based case-control study of 250 Polish incident breast cancer cases (49.2% of eligible) and 301 (41.4% of all selected) frequency matched for age controls was conducted in 2003-2004 in the Region of Western Pomerania. Women were asked to compare their total physical activity at ages 10-12 years and 13-15 years with the activity of their female peers by choose from one of three categories: 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 method of maximum likehood. Women who reported having been physically more active than their peers at ages 10-12 years had an age-adjusted OR=0.88 (95% CI=0.36-2.15, P for trend=0.37) as compared with those reported being less active. 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 multivariate adjusted ORs were also decreased among women who were at least such active as their peers, but the reductions were not statistically significant. For women who were more physically active than their peers during both age periods the adjusted OR was 0.30 (95% CI=0.11-1.34, P for trend =0.21). These results show no protective role for physical activity in childhood on breast cancer development among women aged 35-75 years. Further investigations employing larger sample sizes with comprehensive assessment of physical activity during the childish years are needed to verify this evidence.