Study summary: The present prospective study was designed to investigate the association between intake of conjugated linoleic acids (CLAs) and the risk of breast cancer. Animal and in vitro studies suggest that CLAs, fatty acids found primarily in dairy products and in the meat of ruminants, have protective activities against mammary carcinogenesis. However, findings from population studies in relation to breast cancer risk are few and inconsistent. The study included 61,433 women in the Swedish Mammography Cohort who were cancer-free at enrollment during 1987 to 1990. Dietary CLA intake was determined by means of a food-frequency questionnaire at baseline. Cox proportional hazards models were used to calculate rate ratios (RRs) and 95% confidence intervals (CIs), adjusted for breast cancer risk factors. During a mean follow-up period of 17.4 years, 2,952 new cases of invasive breast cancer were diagnosed in the study group. No significant association was found between dietary CLA intake and risk of breast cancer, overall or by estrogen receptor (ER) and progesterone receptor (PR) status of the tumors. The multivariate relative risks (95% CI) for the highest fifth of CLA intake (155.7 mg/day) compared with the lowest quintile (<78.1 mg/day) were 1.04 (0.92 - 1.17) for overall breast cancer, 1.09 (0.90 - 1.31) for ER+/PR+ tumors, 1.09 (0.78 - 1.53) for ER+/PR- tumors, and 0.84 (0.57 - 1.24) for ER-/PR- tumors. The authors conclude that the results provide no evidence of a protective effect of CLA against breast cancer development in women.