Interest in the relationship between health behaviours and academic achievement has recently intensified in the face of an epidemic of childhood and adolescent obesity and converging school reforms in the United States and other nations with advanced economies. Epidemiologic research has demonstrated that poor diet and lack of adequate physical activity place children at risk for being overweight and obese and thus influence future health status. Additional research has also shown that children and adolescents whose diets are nutritious and whose participation in physical activity is high tend to perform better on various measures of cognitive performance and academic achievement. We analysed cross-sectional survey data from 5810 Icelandic school children to explore the relationship between selected health behaviours and academic achievement. Body mass index, diet and physical activity explained up to 24% (P less than 0.01) of the variance in academic achievement when controlling for gender, parental education, family structure and absenteeism. Variance explained increases to 27% when depressed mood (P less than 0.05) and self-esteem (P less than 0.01) are added to the model, but confounds the role of physical activity. Although not robust, these findings are consistent with previous work and affirm the complexity of the relationship of health to academic achievement. (Contains 4 tables and 1 figure.)