The validity and reproducibility of a self-administered dietary questionnaire has been tested with specific attention to differences between obese and non-obese subjects. To test the validity, the dietary questionnaire was compared with 4-day food records, 24-h energy expenditure (24EE) and nitrogen excretion in 45 obese and 19 non-obese men and women. Energy intake was 2% higher (non-significantly) from questionnaire than from food records in the non-obese, but 35% higher (P < 0.001) in the obese. Comparing energy intake from the questionnaire with estimated 24EE, the questionnaire gave 4% higher values in both the non-obese and obese, differences which were not significant. The reproducibility in the obese sample that completed the questionnaire twice was comparable to that observed in normal populations. These data suggest that it is possible to obtain information on obese subjects' dietary intake that is at least as valid and reproducible as that from normal weight individuals.