Ten probands with moderate to severe developmental delay were found to have a supernumerary inv dup (15) chromosome. These patients and their families were studied by both cytogenetic and molecular methods. Cytogenetic polymorphisms associated with the 15p short arm suggested a maternal derivation for the marker chromosome in all informative cases. One marker was directly maternally inherited. Molecular analysis employing Southern blotting and polymerase chain reaction (PCR) of microsatellite repeats demonstrated the presence of extra alleles in the 15q11q13 region. All ten of the probands demonstrated an extra band at one or more locus without recourse to densitometry. All of the inv dup (15) markers were comparable in size to a G group chromosome but there were differences in the positions of the breakpoints in 15q. There was an inconsistent relationship between marker size, gene dosage and severity of phenotype.