SummaryBackground Questionnaires are widely used in epidemiological studies to measure eczema symptom prevalence, but there are concerns regarding their accuracy if used as a diagnostic tool. Objectives To compare the performance of a validated eczema symptom questionnaire and a standardized skin examination protocol employed in the second phase of the International Study of Asthma and Allergies in Childhood (ISAAC). Methods A total of 30 358 schoolchildren aged 8-12 years from 18 countries were examined for flexural eczema. Parents also completed an eczema symptom questionnaire. We compared prevalence estimates at the population level based on the questionnaire vs. physical examination. We also compared the skin examination and the ISAAC questionnaire in making a diagnosis of flexural eczema. Results The point prevalences for flexural eczema at centre level based on a single examination were lower than the questionnaire-based 12-month period prevalences (mean centre prevalence 3·9% vs. 9·4%). Correlation between prevalences of both outcome measures was high (r = 0·77, P < 0·001). At the individual level, questionnaire-derived symptoms of 'persistent flexural eczema in the past 12 months' missed < 10% of cases of flexural eczema detected on physical examination. However, between 33% and 100% of questionnaire-based symptoms of 'persistent flexural eczema in the past 12 months' were not confirmed on examination. Conclusions ISAAC questionnaire-derived symptom prevalences are sufficiently precise for comparisons between populations. Where diagnostic precision at the individual level is important, questionnaires should be validated and potentially modified in those populations beforehand, or a standardized skin examination protocol should be used.