Breast cancer incidence is low in India compared with high-income countries, but it has increased in recent decades, particularly among urban women. The reasons for this pattern are not known although they are likely related to reproductive and lifestyle factors. Here, we report the results of a large case-control study on the association between breastfeeding and breast cancer risk. The study was conducted in 2 areas in South India during 2002-2005 and included 1,866 cases and 1,873 controls. Detailed information regarding menstruation, reproduction, breastfeeding and physical activity was collected through in-person interview. Odds ratios (OR) and 95% confidence intervals (CI) were estimated by unconditional logistic regression models. Breastfeeding for long duration was common in the study population. Lifetime duration of breastfeeding was inversely associated with breast cancer risk among premenopausal women (p-value of linear trend, 0.02). No such protective effect was observed in postmenopausal women, among whom a protective effect of parity was suggested. A reduction of breast cancer risk with prolonged breastfeeding was shown among premenopausal women. Health campaign focusing on breastfeeding behavior by appropriately educating women would contribute to reduce breast cancer burden.