We report on a man with mental retardation and a complex karyotype with cells containing up to three morphologically distinct supernumerary marker chromosomes (SMCs) in most metaphases. Fluorescence in situ hybridisation studies using chromosome 15‐specific probes characterised the presence of seven SMCs all derived from chromosome 15. The results suggest that the patient originally had a large inv dup(15) containing two copies of the Prader‐Willi/Angelman critical region which became mitotically unstable, and by a process of dynamic mosaicism various morphologically distinct SMCs arose.