A total of 5211 schoolchildren aged 10-15 years participated in an Australia-wide sample survey conducted in 1985 and completed dietary and demographic assessment, and the measurement of body mass index: 1017 children aged 12 and 15 years gave blood for serum lipid analysis. Group mean differences in serum lipids and body mass index with age, sex, socioeconomic status and ethnic origin were determined. Nutritional analysis generated group mean values for daily energy intake, per cent kilo-joules from total fat, saturated fat, monosaturated fat and polyunsaturated fat, as well as the polyunsaturated:saturated fat ratio and nutrient density of fibre. In multiple regression analyses, socioeconomic and gender based differences in serum lipids could be explained by differences in diet, whereas age group differences could not. Although there were statistically significant differences in dietary fat intake on the basis of ethnic origin, these were not reflected in differences in serum lipids. For girls, dietary fat variables were more important predictors of serum lipids than body mass index. For boys, the reverse was true. These results suggest that class differences in cardiovascular risk arise from dietary differences that are present from an early age. Sex-based differences in serum lipids seem to reflect different mechanisms in girls and boys. In the former, dietary differences are of importance but for the latter, anthropomorphic (possibly exercise-linked) differences are the most important. These findings imply that cardiovascular risk preventive programmes for children need to take into account the mechanisms of social inequality and sex-based differences.