A 4 year old female referred with developmental delay was found to have two de novo abnormal derivatives of chromosome 15, a supernumerary inverted duplicated marker chromosome (inv dup (15)) and an interstitial triplication of proximal 15qll-ql3 or 14 in one ofthe two 15 homologues (trip (15)). Fluorescence in situ hybridisation (FISH) using probes within and flanking the Prader-WillilAngelman syndrome critical region (PWASCR) confirmed the triplication in the abnormal 15 homologue. The inv dup(15) was dicentric, positive for IR39d which maps proximal to the PWASCR, but was negative for all the PWASCR FISH probes used. Results using polymorphic microsatellite repeats confirmed that the additional material in the trip(l5) was maternal in origin and included several PWASCR loci. The presence of two de novo abnormalities involving the proximal region of 15q suggests a linked mechanism of origin. (JMed Genet 1998;35:425-428)