Objective: the objective of the study was to assess the psychometric properties of the Brief Illness Perception Questionnaire in a sample of adolescents with chronic endocrine or pneumological conditions and to analyze the dimensionality and reduce the scale elaborating scales by sex and medical diagnosis. Method: we evaluated 510 patients aged 9-16 years using the Brief Illness Perception Questionnaire and the Hospital Anxiety and Depression Scale. We carried out tests of reliability, construct and criterion validity and a comparison of means based on the diagnosis and socio-demographic variables. The reliability and validity analyses showed adequate psychometric properties for this scale, with better results obtained for a single dimension after eliminating 3 items. Results: adolescents with type 1 diabetes and girls were found to have an increased threat perception of their illness. Anxiety/depression was positively associated with the perception of illness. Conclusion: this questionnaire is a useful and practical tool for evaluating adjustment to illness in pediatric patients.