We report changes in renal function and growth rate in children with reduced renal function who were kept on a low normal phosphorus formula until 18 months of age. The relationship between serum creatinine values and derived creatinine clearance and residual renal function is reviewed regarding the risk of progressive loss of function. Expressing growth as growth velocity standard deviation scores over 6-month intervals gave a more accurate description of growth performance than did change in height standard deviation scores. A relationship between residual clearance and growth velocity was inferred. However, the growth of children with a less severe reduction in function was affected intermittently by intercurrent infections and inadequately treated acidosis and salt wasting. Children with a more severe reduction in function, despite maintenance on the low-phosphorus formula, had elevated serum parathormone levels. We conclude that following growth (as serial growth velocity standard deviation scores) in relation to other variables gives more insight into factors affecting growth in these children; the level of residual function affects growth potential but other complicating factors also have an effect.