Persistent organochlorine compounds (OCs) is a mixture of persistent and lipophilic chemicals that bioaccumulate in the environment and includes organochlorinated pesticides (OCPs), dioxins, and polychlorinated biphenyls (PCBs). Studies have linked OC exposure to negative health outcomes, mainly relying on biomarkers for the exposure assessment. However, the high costs associated with using biomarkers limits statistical power and the detection of relationships with disease that have low incidence rates. In such situations, assessment of OC exposure in prospective cohort studies using food frequency questionnaire (FFQ) data can be used instead if reasonable validity is proven. In a previous study, published in 2012, FFQ-based estimates of concurrent, past, and long-term dietary exposure to PCB-153 was validated against concentrations of six individual PCB congeners in serum of 201 women from the population-based Swedish Mammography Cohort-Clinical (SMC-C)3. Following adjustment for body weight and age, as well as within-person variability intraclass correlation of FFQ-based PCB, the Spearman correlation coefficients for long-term FFQ-based PCB estimates ranged between 0.30-0.58. Since this was published the number of women in the SMC-C whose samples have been analysed for OCs has increased to 742, and the number of OC congers assessed to 181. In the proposed project we plan to perform an updated validation study that include the six congers that have already been validated previously but in a larger sample. In addition, we will validate the additional 12 OCs that have been measured against the FFQ-based estimated intake.