The purpose of this study was to determine the effect of the fragile X syndrome on limb (upper limb length) and facial (ear length) measurements in relation to body height, which was considered as a covariate. The analysis was performed in 52 fragile X Caucasian families, including affected individuals as well as their normal relatives. The maximum likelihood analysis of complex pedigrees, under the assumption of multivariate normality, was used to select the best genetic model and to estimate model parameters. This method allowed us to test various assumptions concerning the effect of normal hereditary variation, as well as of genetic anomaly and other relevant factors, in the mean and variance of a quantitative trait, and to obtain the maximum likelihood parameters for these effects. The results demonstrated that, if the family factor is accounted for in a model, the fragile X condition affects growth of both upper limb length and ear length, disproportionately to body height.