To evaluate the relationship between the growth hormone 1 (GH1) T1663A polymorphism, recreationalphysical activity and body mass index (BMI) with reference to breast cancer, we conducted a case-control studywith 669 cases of breast cancer and 682 population-based controls in Jiangsu Province, China. A structuredquestionnaire was used to elicit detailed information. All subjects completed an in-person interview. GH1genotypes were identified using PCR-RFLP methods. Odds ratios (ORs) were estimated with an unconditionallogistic model. The distribution of GH1 genotypes was not significantly different between controls and cases(χ2=2.576, P=0.276). Results of stratified analysis by the participation status of the recreational physical activityshowed that the persons with GH1 A allele were at a decreased risk of breast cancer (adjusted-OR=0.66; 95%CI, 0.50-0.87) only among inactive individuals. Stratified analysis by BMI showed that the genotype A/A wasassociated with a decreased risk of breast cancer only among individuals of the BMI <25 (adjusted-OR=0.80;95% CI, 0.66-0.98). The findings of this study suggest that recreational physical activity and BMI may modifyany association between the GH1 T1663A polymorphism and breast cancer risk.