We have studied a patient with Angelman syndrome (AS) and a 47,XY,+inv dup(15) (pter-->q11::q11-->pter) karyotype. Molecular cytogenetic studies demonstrated that one of the apparently normal 15s was deleted at loci D15S9, GABRB3, and D15S12. There were no additional copies of these loci on the inv dup(15). The inv dup(15) contained only the pericentromeric sequence D15Z1. Quantitative DNA analysis confirmed these findings and documented a standard large deletion of sequences from 15q11-q13, as usually seen in patients with AS. DNA methylation testing at D15S63 showed a deletion of the maternally derived chromosome 15q11-q13 on one of the apparently cytogenetically normal 15s, and not by the presence of an inv dup(15). This is the fourth patient with an inv dup(15) and AS or Prader Willi syndrome, who has been studied at the molecular level. In all cases an additional alteration of chromosome 15 was identified, which was hypothesized to be the cause of the disease. Patients with inv dup(15)s may be at increased risk for other chromosome abnormalities involving 15q11-q13.