The purpose of this study was to examine relationships between the longitudinal development of biological risk factors for coronary heart disease (CHD) (namely, serum lipids, body fatness, blood pressure and cardiorespiratory fitness) and the development of lifestyles considered to be atherogenic. The study was conducted in a representative sample of adolescents from Northern Ireland, a region of high coronary mortality, and atherogenic aspects of lifestyle such as cigarette smoking, physical inactivity and selected aspects of diet were examined. Repeated measurements were made at 12 and 15 y of age on 229 boys and 230 girls. Longitudinal relations were analysed with generalised estimating equations, and the following longitudinal relations were found: for boys diastolic blood pressure was positively associated with vitamin C intake (P = 0.014), and inversely with energy intake (P = 0.006), and smoking (P = 0.048). Systolic blood pressure was inversely related to physical activity (P = 0.012), and smoking (P = 0.000). Body fatness was also inversely related to smoking (P = 0.006). Total cholesterol (TC) was positively related to physical activity (P = 0.044) and the TC:HDL cholesterol ratio positively to vitamin C intake (P = 0.008). Cardiorespiratory fitness was positively related to physical activity (P = 0.000) and inversely to smoking (P = 0.031). For girls, systolic blood pressure was positively related to vitamin C intake (P = 0.042); HDL cholesterol inversely to carbohydrate intake (P = 0.014), fat intake (P = 0.031), cholesterol intake (P = 0.042) and smoking (P = 0.035) and positively to energy intake (P = 0.035). The TC:HDL cholesterol ratio was inversely related to energy intake (P = 0.038) and finally, cardiorespiratory fitness positively to physical activity (P = 0.001). These results offer additional evidence that changes in lifestyle are associated with changes in biological risk factors in adolescents. Education and intervention at this stage, particularly in relation to cigarette smoking, physical activity and certain aspects of diet appear justified as part of a preventative strategy for CHD.