OBJECTIVE--To assess the nutritional status of children with congenital heart disease. DESIGN--Six anthropometric, 24 biochemical, and five haematological markers of nutritional wellbeing were measured in children with congenital heart disease. SETTING--The west of Scotland. PATIENTS--48 children admitted consecutively for surgical correction of congenital heart disease. MAIN OUTCOME MEASURES--Height, weight, and triceps and subscapular skin fold thicknesses were considered abnormal if they were below the third centile compared with standard reference data for age matched British children. Mid-arm circumference and arm muscle circumference were considered abnormal if they fell below the fifth centile compared with standard data. Biochemical and haematological data were compared with age matched and locally validated laboratory normals. RESULTS--A marked degree of undernutrition was evident in all children; 52% had weight less than the third centile, 37% were below the third centile for height, and 12.5% were below the third centile for triceps skin fold thickness and 18.8% for subscapular skin fold thickness. Mid-arm circumference and arm muscle circumference were below the fifth centile in 20.1% and 16.7% of children respectively. Five or more of the 29 biochemical and haematological measurements were abnormal in 83.3% of patients; 10 or more were abnormal in 12.5% of patients. CONCLUSIONS--Children with congenital heart disease are frequently undernourished, irrespective of the nature of cardiac defect and the presence or absence of cyanosis.