Objective <p>Dietary intake and sub-optimal nutrition have a significant impact on clinical risk, outcomes and mortality in people with an eating disorder. Despite the range of dietary patterns and nutritional issues, there is limited evidence to guide effective use of dietary assessment methods. The aim of this pilot study was to examine the validity of the diet history against routine nutritional biomarkers in female adults with an eating disorder attending a regional outpatient service.</p> Method <p>This secondary data analysis utilised demographics, nutrient intakes from diet history and nutritional biomarker data. Spearman’s rank correlation, simple and quadratic weighted kappa statistics and the Bland-Altman method were used to explore validity of the diet history compared to nutritional biomarkers.</p> Results <p>Thirteen female participants (median age 24 years; median BMI 19&#xa0;kg/m²) with an eating disorder were included. Energy-adjusted dietary cholesterol and serum triglycerides showed moderate agreement (simple kappa K = 0.56, <i>p</i> = 0.04), and dietary iron and serum total iron-binding capacity showed moderate-good agreement (simple kappa K = 0.48, <i>p</i> = 0.04; weighted kappa K = 0.68, <i>p</i> = 0.03). Dietary iron and serum total iron-binding capacity were only significantly correlated when dietary supplements were included (<i>r</i> = 0.89, <i>p</i> = 0.02). Dietary estimates of protein and iron improved with larger intakes.</p> Conclusions <p>This pilot study suggests the diet history may be useful for assessing dietary intake in people with an eating disorder and highlights the importance of targeted questioning around dietary supplement use. Determining the validity of dietary assessment methods across ages, diagnoses and settings is required.</p>