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Background: The Food Frequency Questionnaire (FFQ) is the most commonly used method for ranking individuals based on long term food intake in large epidemiological studies. The validation of an FFQ for specific populations is essential as food consumption is culture dependent. The aim of this study was to develop a Semi-quantitative Food Frequency Questionnaire (SFFQ) and evaluate its validity and reproducibility in estimating nutrient intake in urban and rural areas of Argentina. Methods/Principal Findings: Overall, 256 participants in the Argentinean arm of the ongoing Prospective Urban and Rural Epidemiological study (PURE) were enrolled for development and validation of the SFFQ. One hundred individuals participated in the SFFQ development. The other 156 individuals completed the SFFQs on two occasions, four 24-hour Dietary Recalls (24DRs) in urban, and three 24DRs in rural areas during a one-year period. Correlation coefficients (r) and deattenuated correlation coefficients between 24DRs and SFFQ were calculated for macro and micro-nutrients. The level of agreement between the two methods was evaluated using classification into same and extreme quartiles and the Bland- Altman method. The reproducibility of the SFFQ was assessed by Pearson correlation coefficients and Intra-class Correlation Coefficients (ICC). The SFFQ consists of 96 food items. In both urban and rural settings de-attenuated correlations exceeded 0.4 for most of the nutrients. The classification into the same and adjacent quartiles was more than 70% for urban and 60% for rural settings. The Pearson correlation between two SFFQs varied from 0.30-0.56 and 0.32-0.60 in urban and rural settings, respectively. Conclusion: Our results showed that this SFFQ had moderate relative validity and reproducibility for macro and micronutrients in relation to the comparison method and can be used to rank individuals based on habitual nutrient intake. [ABSTRACT FROM AUTHOR]