Objectives Current guidance recommends that dental practitioners should routinely give dietary advice to patients, with diet diaries as a tool to help diet assessment. We explored patients' compliance with diet-diaries usage in a paediatric clinic within a teaching hospital setting, where remuneration is not an issue. Objectives were to investigate associated factors affecting diet diaries return rate and the information obtained from returned diaries.Methods A retrospective study of 200 randomly selected clinical records of children aged 5-11 years who had received diet analysis and advice as part of a preventive dental care programme at a dental teaching hospital between 2010 and 2013. Clinical records, with a preventive care pro forma, were included in the study. Data on social and family history, DMFT-dmft, oral hygiene practices, dental attendance and dietary habits were obtained and compared with information given in completed diet-diaries. A deductive content analysis of returned diet-diaries was undertaken using a pre-developed coding scheme.Results Of 174 complete records included in this study, diet diaries were returned in 60 (34.5%) of them. Diet diaries were more likely to be returned by those children who reported that they regularly brushed their teeth (P <0.05), and those who came from smaller families (P <0.05). Content analysis of diet diaries enabled the identification of harmful types of foods and drinks in 100% of diaries. General dietary issues, frequency and between-meals intake of sugars were also all captured in the majority of diaries (95.0%, N = 56). Information on sugar amount (53.0%, N = 32), prolonged-contact with teeth (57.0%, N = 34) and near bedtime intakes (17.0%, N = 28) was reported in fewer diaries.Conclusions The return rate of diet-diaries in this setting was low, and associated with patients' demographic and oral health characteristics. Returned diet-diaries showed a varied range of missing important dietary information, such as sugar amount, which appears to compromise their validity as a diet assessment tool. Development of a more reliable and acceptable dietary assessment tool for use in the dental setting is needed.