Inv dup(15) is a clinically significant bisatellited derivative of chromosome 15. Five unrelated patients with this abnormality are described and compared with ten confirmed and nine suspected cases in the literature. Mental and developmental retardation, hypotonia, behavioral disturbances, seizures, abnormal dermatoglyphics, and mild somatic anomalies were the most consistent findings. The extra chromosomes in our patients were identified with the aid of various techniques, including distamycin A/DAPI banding. A comparison of satellite polymorphisms suggested that the rearrangements frequently arose by meiotic nonsister chromatid exchange and second-division nondisjunction. A maternal origin was indicated in two cases, and parental ages were distinctly elevated.