A de novo bisatellited derivative of chromosome 15, inv dup (15) (pter→q11 or 12::p11 or q11 or 12 →pter), was identified by multiple banding techniques in a patient with Prader‐Willi syndrome. A comparison of familial no. 15 short arm polymorphisms indicated that the extra chromosome was the result of a non‐sister chromatid exchange between the paternal no. 15 homologs prior to or during meiosis.