Cornelia de Lange syndrome is a well-known multiple congenital anomalies/intellectual disability syndrome with genetic heterogeneity and wide clinical variability, regarding the severity of both the intellectual disabilities and the physical features, not completely explained by the genotype-phenotype correlations known to date. The aim of the study was the identification of prognostic features, ascertainable precociously in the patient's life, of a better intellectual outcome and the development of a new prognostic index of severity of intellectual disability in CdLS patients. In 66 italian CdLS patients aged 8 years or more, we evaluated the association of the degree of intellectual disability with various clinical parameters ascertainable before 6 months of life and with the molecular data by the application of cumulative regression logistic model. Based on these results and on the previously known genotype-phenotype correlations, we selected seven parameters to be used in a multivariate cumulative regression logistic model to develop a prognostic index of severity of intellectual disability. The probability of a mild ID increases with the reducing final score less than two, the probability of a severe ID increases with the increasing final score more than three. This prognostic index allows to define, precociously in the life of a baby, the probability of a better or worse intellectual outcome in CdLS patients. © 2016 Wiley Periodicals, Inc.