Prader-Willi syndrome (PWS) is a complex genetic disorder. About 70% of cases have a paternal deletion at 15q11-q13, and most of the remaining cases are caused by maternal uniparental disomy (UPD). In rare cases of PWS with maternal UPD, small marker chromosomes are identified. Patients with inv dup(15) are at an increased risk of developing PWS or Angelman syndrome (AS) due to UPD. They may be also at increased risk for developmental delay due to additional copies of genes located within the PWS/AS critical region. Therefore, molecular investigations in patients with a supernumerary marker chromosome (SMC) are necessary to provide proper genetic counseling. We report a female infant with central hypotonia, weak crying, feeding problems, failure to thrive, and developmental delay after birth. Chromosome analysis revealed an SMC in 55% of metaphase cells. Fluorescence in situ hybridization showed that this marker chromosome was constituted by a small isodicentric inverted duplication of chromosome 15 [inv dup(15)]. Microsatellite analysis showed uniparental isodisomy of maternal chromosome 15 in the proband. Diagnosis of PWS was further confirmed by methylation-specific polymerase chain reaction. The inv dup(15) marker chromosome was also of maternal origin. Follow-up at the age of 18 months revealed a height in the 10th percentile and weight in the 50th percentile. She had poor activity and muscle tone, and was unable to walk independently. There was no psychomotor retardation, behavior disturbance or seizure.