Growth failure is a well recognised consequence of severe congenital heart disease. Total daily energy expenditure (TDEE) was investigated in eight infants with severe congenital heart disease to determine whether an increase in this parameter is an important factor in their failure to thrive, and to estimate the energy intake that would be required to allow normal growth. The infants were studied over a seven day period before surgery using the doubly labelled water method. Growth failure was evident; their mean age standardised body mass index was 80% of the expected value. Mean TDEE was 425 kJ/kg, significantly greater than in healthy infants (mean TDEE/kg SD score = +1.4; 95% confidence interval +0.27 to +2.57). In contrast, their energy intake was only 82% of the estimated average requirements. It was estimated that in early infancy a gross energy intake of 600 kJ/kg/day is required for normal growth in patients with congenital heart disease. This is unlikely to be achieved by energy supplements alone and early recourse to nasogastric feeding should be considered.