The genetically determined neurodevelopmental disorder, Prader-Willi syndrome (PWS), has two main genetic subtypes: a 15q11-q13 deletion affecting the paternally inherited chromosome 15 and chromosome 15 maternal uniparental disomy (mUPD) in which two maternal copies of chromosome 15 are inherited but no paternal copy. It has been accepted that these subtypes occur in approximately 70 and 25% of cases, respectively. This is the first report of a greater proportion (50%) of those with PWS due to mUPD in children presently under 5 years living in the UK. Increasing maternal age at conception is likely to explain the changing proportions in this generation of mothers.