Prevalence of overweight and obesity in UK firefighters may exceed that of the general population (1) . UK Firefighter dietary behavior has recently been investigated in a fire station-based dietary intervention pilot trial (1) , which utilised the EPIC-Norfolk food frequency questionnaire (FFQ) (2) . Although this was able to detect significant dietary changes (1) , participant feedback identified it as burdensome. Furthermore, it was validated for a different population group, therefore its validity for assessing firefighter diets is unknown. We developed and tested the validity and reproducibility of the UK's first firefighter FFQ (FF-FFQ).In June 2019, the EPIC-Norfolk FFQ was completed by 180 London firefighters. The thirty least consumed food items (reported mean intakes of never or less than once/month) were removed. A question on takeaway food consumption was added, as fieldwork suggested takeaways to be regularly consumed. The resulting FF-FFQ amounted to a six-page document (reduced from eleven). This was successfully pre-tested on a firefighter focus group who found it understandable and acceptable. Its relative validity was then tested against three multi-pass 24 hr. recalls, collected over a four-month period (June-November 2019) on a sample of n = 69 firefighters.Correlations between the methods were medium and significant (p < 0.01) for energy (r = 0.42), carbohydrate (r = 0.42), protein (r = 0.42), fat (r = 0.43), fibre (r = 0.49), saturated fat (SFA) (r = 0.45), monounsaturated fat (MUFA) (r = 0.32), vitamin C (r = 0.33) and sodium (r = 0.32), but weak for polyunsaturated fat (PUFA) (r = 0.24, p = 0.05). This is consistent with a review of FFQ validation studies (3) which found correlation coefficients averaged between 0.4-0.5 when comparing a reference method with FFQs which display a specified portion size, but lower (0.2-0.5) when no portion size was specified. Similar to the EPIC-Norfolk FFQ, the FF-FFQ displays clearer portion sizes for some food groups than for others, which may explain the lower correlations for MUFA, PUFA, vitamin C and sodium. Bland-Altman analyses indicated good agreement between methods for energy and each nutrient, with an average of 96% of cases falling between the limits of agreement.The FF-FFQ was also tested for reproducibility (n = 72). Comparisons were made between dietary intakes recorded by the FF-FFQ on two occasions, four months apart. Correlations were strong and significant (p < 0.01) for energy (r = 0.75), carbohydrate (r = 0.8), protein (r = 0.62), fat (r = 0.69), SFA (r = 0.66), MUFA (r = 0.71), PUFA (r = 0.7), fibre (r = 0.71), vitamin C (r = 0.69) and sodium (r = 0.74). Bland-Altman analyses also indicated good agreement between administrations indicating good precision, suggesting usefulness for detecting temporal dietary changes.Showing reproducibility along with an overall acceptable level of relative validity, the FF-FFQ has demonstrated utility for its primary uses as both a screening tool to indicate high/low intakes of certain foods, and as a m...