The effects of body weight and age on serum cholesterol, serum triglycerides, serum urate and systolic blood pressure were examined in 600 male and 400 female blood donors aged 20 to 49 years. In the men significant correlations with body mass index were found for all four variables in each decade. In the women below 40 only the correlation with blood pressure was significant. In the fifth decade the correlations resembled those in the men, save for triglycerides. After adjusting for weight, age had no independent influence on the prevalence of hypertriglyceridaemia or hypertension in either sex. In men the effect of body weight on the prevalence of hypercholesterolaemia was age dependent. Age influenced hyperuricaemia independently of weight. In women only serum cholesterol was affected by age after allowing for weight. There was marked clustering of high values of the four variables in the heavier men and women and this increased with age. The leanest men and women were remarkably free of high values.