Lifestyle factors related to breast cancer risk were examined in a case-control study nested in a cohort in Karunagappally, Kerala, South India. We sought interviews with all the residents in Karunagappally with the population of 385,103 (191,149 males and 193,954 females) in the 1991 census and established a cohort of 359,619 (93% of the population in 1991) in 1990. For analysis 264 breast cancer cases with age > or = 20 years were selected from 438 breast cancer cases reported during the period 1990-2004 and for each case 3 non-cancer controls were randomly selected matched for age, religion and place of residence through the Cancer Registry, Karunagappally. Conditional logistic regression was used for the analysis. In the present study, in addition to a low number of pregnancies (P <0.001 and P for trend <0.001), more frequent intake of roots and tubers except tapioca (cassava) (OR for > or = 5 times =1.56, 95% CI=1.09, 3.09, P for trend <0.05), milk drinking (OR=1.78, 95% CI=1.17-2.69, P<0.01) and consumption of chicken meat (OR=1.84, 95% CI=1.09-3.09, P<0.05) were found to increase breast cancer risk. The present study further showed that consumption of tapioca which is a commonly used food item in South India, particularly in Kerala, reduced breast cancer risk (OR=0.55, 95% CI=0.37-0.83, P<0.01). Risk analysis was attempted among pre- and post-menopausal women separately and similar odds ratio were obtained. Consumption of tapioca (cassava) decreased risk of developing breast cancer among premenopausal women (P<0.001 and OR=0.35, 95% CI=0.18, 0.65) and a low number of pregnancies (P<0.01), consumption of roots & tubers (P<0.05), usage of chicken meat (P=0.05) increased the risk of breast cancer among post-menopausal women. Further studies seem warranted to confirm the possible protective effect of tapioca consumption. There is an increasing need of breast cancer prevention programs responsive to the cultural practices of the people and the study results should provide leads to cancer control programs especially in rural areas.