A 3-year-old female referred with developmental delay, hypotonia and seizures was found to have a cryptic interstitial duplication of the Prader-Willi/Angelman critical region (PWACR). Her clinical features form part of a common phenotype characteristic of PWACR duplications including developmental delay, behavioural problems and speech difficulties. Microsatellite analysis showed that the duplication had arisen de novo, was maternal in origin and involved the entire 4-Mb PWACR between the common deletion breakpoints. The existence of cryptic rearrangements emphasises the need for molecular tests alongside conventional cytogenetics when investigating abnormalities involving this imprinted region.