OBJECTIVES--(a) To determine the excess mortality from all causes and from coronary heart disease in patients with familial hypercholesterolaemia; (b) to examine how useful various criteria for selective measurement of cholesterol concentration in cardiovascular screening programmes are in identifying these patients. DESIGN--Prospective cohort study. SETTING--Eleven hospital outpatient lipid clinics in the United Kingdom. PATIENTS--282 men and 244 women aged 20-74 with heterozygous familial hypercholesterolaemia. MAIN OUTCOME MEASURE--Standardised mortality ratio, all adults in England and Wales being taken as standard (standardised mortality ratio = 100 for standard population). RESULTS--The cohort was followed up for 2234 person years during 1980-9. Fifteen of the 24 deaths were due to coronary heart disease, giving a standardised mortality ratio of 386 (95% confidence interval 210 to 639). The excess mortality from this cause was highest at age 20-39 (standardised mortality ratio 9686; 3670 to 21,800) and decreased significantly with age. The standardised mortality ratio for all causes was 183 (117 to 273) and also was highest at age 20-39 (standardised mortality ratio 902; 329 to 1950). There was no significant difference between men and women. Criteria for measurement of cholesterol concentration in cardiovascular screening programmes (family history, presence of myocardial infarction, angina, stroke, corneal arcus, xanthelasma, obesity, hypertension, diabetes, or any of these) were present in 78% of patients. CONCLUSIONS--Familial hypercholesterolaemia is associated with a substantial excess mortality from coronary heart disease in young adults but may not be associated with a substantial excess mortality in older patients. Criteria for selective measurement of cholesterol concentration in cardiovascular screening programmes identify about three quarters of patients with the clinically overt condition.