Department of Nutritional Epidemiology, TNO Nutrition and Food Research, Zeist, Netherlands. voorrips@voeding.tno.nl BACKGROUND: Conjugated linoleic acid (CLA), which is present in milk products and meat from ruminants, appears to have anticarcinogenic activity against breast cancer in animal and in vitro experiments. To date, few epidemiologic data are available in humans. OBJECTIVE: This study evaluated the relation between intakes of CLA and other fatty acids and breast cancer incidence in the Netherlands Cohort Study. DESIGN: Intake data derived from a validated 150-item food-frequency questionnaire were linked to an existing database with analytic data on specific fatty acids in European foods (the TRANSFAIR study). With 6.3 y of follow-up and 941 incident cases of breast cancer, multivariate rate ratios and 95% CIs were calculated for energy-adjusted intakes of fatty acids and CLA-containing food groups (eg, butter, cheese, milk, other milk products, and meat). RESULTS: CLA intake showed a weak, positive relation with breast cancer incidence (rate ratio for highest compared with lowest quintile: 1.24, 95% CI: 0.91, 1.69; P for trend = 0.02). Statistically significant positive associations were found with total trans fatty acids and (borderline) with saturated fatty acids. Significant inverse associations were found with monounsaturated and cis unsaturated fatty acids, whereas total fat and energy intake of CLA-containing food groups were not related to breast cancer incidence. CONCLUSION: The suggested anticarcinogenic property of CLA in animal and tissue culture models could not be confirmed in this epidemiologic study in humans.