To develop accurate weight for age charts for individuals with achondroplasia. These novel weight for age, gender-specific growth curves for achondroplasia patients from birth through 16 years were constructed from a longitudinal, retrospective, single observer cohort study of 334 individuals with achondroplasia. Weight for age data from 301 subjects in this achondroplasia cohort, constituting 1,964 total weight measurements, are presented in these weight for age curves. Percentiles (5, 25, 50, 75, 95th) were estimated across the age continuum by gender, using a 1 month window (±0.5 months) around each time point of interest. Percentiles were smoothed using a quadratic, penalized smoother by a semi-parametric model approach. Raw weight data from the achondroplasia cohort are compared to that of average stature children presented in the current CDC growth curves, divided into 0–36 months and 2–16 years. There was overlap of birth weight between achondroplasia and average stature infants. This statistical modeling method can be applied to other anthropometric parameters collected from this achondroplasia cohort (e.g., length, BMI), other skeletal dysplasia diagnoses, and to syndromic, non-skeletal dysplasia diagnoses which may benefit from standardization of weight for age. © 2007 Wiley-Liss, Inc.