The acrocephalosyndactylies are a group of conditions with a wide variability of expression. We report a patient with features of the Pfeiffer syndrome (acrocephalosyndactyly type V), initially believed to be a new mutation in her family. Further examination of her mother demonstrated abnormalities of her right thumb which we believe may represent mild expression of the Pfeiffer syndrome. This observation illustrates the variability of phenotypic expression in the Pfeiffer syndrome and underscores the importance of careful investigation of parents and other family members when evaluating individuals with this condition.