Children with cystic fibrosis (CF) frequently have growth retardation. Standard growth velocity is dependent on an adequate supply of energy. The effect of dietary counselling and high energy food supplementation on the growth of eight children (aged 4 to 9 years) with CF was determined. The 2-year study was divided into control and test periods each lasting 1 year. Only during the test period did the mean energy intake reach or exceed 100% of the recommended energy requirements. When energy intake increased, weight growth increased significantly (p less than 0.05). Even with pancreatic enzyme replacement, the CF subjects had increased fecal energy losses. Thus, since dietary energy intake would overestimate energy available for metabolism, absorbed energy was determined. There was a positive linear relationship between absorbed energy and weight gain (p less than 0.01). Normal rates of weight gain were seen once absorbed energy intakes reached 100-110% of requirements. The children grew in height and weight at normal rates during the test period, whereas they showed retarded growth during the control period. Our data provide evidence that children with CF have inadequate energy intakes. Furthermore, we have shown that absorbed energy intake is the principal factor limiting the growth of CF children in the age group studied. Finally, it is possible to promote normal growth in CF children with dietary counselling and high energy supplements.