BACKGROUND: The use of similar standardised measures of psychopathology for population surveys permits cross-cultural comparisons. However, interpretation of findings can be challenging because rating thresholds may differ across cultures. By combining questionnaire and interview data, we explore whether lower questionnaire scores in Norway as compared to Britain reflect genuine differences in child mental health, or simply different reporting thresholds. METHODS: Information from the Strengths and Difficulties Questionnaire (SDQ) and the Development and Well-Being Assessment (DAWBA) interview were compared across recent population surveys in Norway and Britain. The Norwegian study (2002-03) had questionnaire data for 6,658 and interview data for 1,024 8-10-year-old children. The British dataset included questionnaire and interview data for 4,898 children of the same age range from two independent surveys (1999 and 2004). RESULTS: Norwegian children had lower SDQ scores on all problem scales (emotional, behavioural, hyperactive and peer relationship) according to parents as well as teachers. DAWBA information showed that the Norwegian prevalence of externalising disorders (behavioural and hyperactivity) was about half that found in Britain, whereas rates of emotional disorders were similar. Norwegian and British children with non-emotional disorders had similar questionnaire scores and rates of problem-recognition by parents and teachers. By contrast, questionnaire scores and problem-recognition were all lower in Norwegian children with emotional disorders. CONCLUSIONS: Lower Norwegian questionnaire scores for externalising problems appear to reflect real and substantial differences between the two countries. By contrast, lower questionnaire scores for emotional problems seem to reflect under-reporting/under-recognition by Norwegian adults, and not a genuinely lower prevalence of emotional disorders. This illustrates that cross-cultural differences in psychopathology based only on questionnaire data may be misleading. Nevertheless, careful use of questionnaire and interview data can focus mental health research on cross-cultural variations likely to reflect genuine health differences.