Longitudinal recumbent length data on 65 cystic fibrosis (CF) patients from 3 months to 6 years of age were used to fit two mathematical models of early childhood growth. The results were compared to published data from the Longitudinal Studies of Child Health and Development at the Harvard School of Public Health. The median values for girls with CF indicate essentially normal growth in the first year of life, whereas CF boys are slightly smaller than normal boys. Both sexes show declines in length relative to normal children in later years. This pattern is reflected in significant deviations from normal values in growth curve parameters for CF patients. While the linear Reed model gave a slightly better fit to the data, the non linear Jenss model allowed easier interpretation of parameters and comparisons with the Boston children. Early childhood growth in cystic fibrosis is characterized by fairly normal exponential growth in the first year of life followed by an earlier approach to a linear asymptote than is found in normal children. Although both the raw data and curve parameters were much more variable in CF patients than in the Boston sample, there were no significant differences in growth between patients diagnosed in infancy because of disease symptoms as opposed to those diagnosed because of family history alone. The appearance of growth retardation in CF primarily after infancy suggests that the cumulative effect of chronic digestive or pulmonary malfunctions, rather than a basic metabolic defect, is responsible for much of the growth retardation seen in these children.