Recent epidemiological studies have suggested passive smoking as possible risk factors in breast cancer etiology. However, the evidence is not conclusive. We conducted a meta-analysis of the risk of breast cancer associated with passive smoking. Eleven studies were identified through Medline, Cancer Abstracts and abstract presented at related scientific meetings. Combined estimates of relative risks (RR) were calculated using fixed and random effect models. Significant heterogeneity was detected among the studies (chi2 = 34.6, p < 0.01). The combined estimate of RR for ever exposure to passive smoking was 1.41 (95% confidence interval (CI): 1.14-1.75). The combined estimate for published studies was 1.43 (95% CI: 1.10-1.85). Only two studies reported significant dose-response relation between level of passive smoking and premenopausal breast cancer. Our analysis suggests a possible weak association between passive smoking and breast cancer. More studies are needed to establish whether the observed association is causal.