The association between obesity and diet is somewhat conflicting. A population study was undertaken to examine factors influencing diet reporting and the possible differences in diet reporting in relation to such factors. Data on diet, fatness, fat distribution, physical activity, smoking, subjective weight problems, slimming history, social class, education and five year weight change were collected from an age and gender stratified sub-group of Danish men (n = 199) and women (n = 201). Diet reporting was estimated as the difference between protein intake calculated from the diet (dietary interview) and from the analysis of 24 h urine nitrogen excretion. Fatness was estimated from impedance measurements, and fat distribution from body circumference measurements. All other variables were assessed by questionnaire. The study showed that diet reporting was dependent on fatness but independent of gender and age. The fattest tertile under-reported their intake by between 20-25%. The dieters, those with subjective weight problems and those who had not been weight stable since the age of 25, under-reported their intake more than others, independent of age and gender. Men gaining or losing more than 3 kg in five years were furthermore found to under-report their intake independent of age, gender and obesity. No effects of smoking, fat distribution, physical activity, social class or education were found. The results of the present study may influence the interpretation of dietary data from other studies.