We describe the first de novo inverted duplication of 18q. Due to the difficulty of identifying de novo chromosome abnormalities based solely on cytologic studies, precise definition of the 18q duplication was attempted by integrating cytogenetic and clinical findings with biochemical and molecular dosage studies. The combined results demonstrated that the proposita had a duplication of 18q21-->q22 with a karyotype of 46,XX,-18, + inv dup(18) (pter-->q12.1::q22-->q21::q12.1-->qter). The duplication of this specific chromosome region does not result in the typical 18 phenotype, supporting the hypothesis that various loci on chromosome 18 may interact to produce the manifestations of this syndrome.