Objective To assess the reliability and relative validity of a diet index score derived from a Short Food Survey (SFS). Design The thirty-eight-item SFS was designed to assess recent dietary intake of 4-11-year-olds to enable calculation of the Dietary Guideline Index for Children and Adolescents. Reliability was assessed based on two online administrations of the SFS, one week apart. Relative validity was assessed by comparing intakes derived from the SFS with those from the mean of three 24 h recalls. Intra-class correlations, Bland-Altman plots and estimated biases were assessed. Cohen's κ coefficients were used to determine the level of agreement between the two methods. Setting Adelaide, Australia. Subjects Sixty-three parents reported on their children's intake (mean age 7·1 (sd 2·1) years). Results The intra-class correlation for reliability ranged from 0·43 for dairy foods to 0·94 for beverages, and was 0·92 for total diet index score (all P < 0·01). The intra-class correlation for validity ranged from 0·04 for meat and alternatives to 0·41-0·44 for fruit, beverages and extra foods, and was 0·44 for the total diet index score. The SFS overestimated the mean diet index score by 16 %, and the bias was consistent across levels of compliance. The percentage agreement into tertiles of index scores was 84% between the administrations of the two SFS, but only 43 % when comparing the SFS with the mean of the recalls. Conclusions The SFS can provide a consistent estimate of overall compliance to dietary guidelines for children aged 4-11 years, but overestimated the total diet index score by 16 % across all levels of compliance.