The adolescent spurt in 230 children with cystic fibrosis (CF) treated at the Cleveland CF centre in northeastern Ohio was compared to that found in normal children from the Fels Longitudinal Growth Study in southwestern Ohio. The Preece-Baines Model 1 (PB1) growth equation was applied to longitudinal height data from both samples to describe a large number of data points for each child in terms of a few biologically meaningful parameters, such as age, height, and velocity at the take-off and peak of the adolescent growth spurt. The growth spurt is delayed by an average of 0.8 years and is about 1 cm/year slower at its peak in CF patients compared to the normal controls. This delay should be considered when comparing clinical growth measurements of adolescent CF patients with normal standards. Peak velocity is lower than expected, even for late-maturing normal children, and height at take-off, peak velocity, and adulthood is significantly reduced, especially in boys. These findings are consistent with the clinical impression that the growth spurt is delayed and attenuated in CF patients, particularly those with poor pulmonary function. Girls homozygous for the delta F508 mutation have significantly more growth retardation than those with other CF mutations.