We report on a 5-year-old girl with Pallister–Killian syndrome (OMIM #601803) due to mosaicism of two supernumerary isochromosomes (hexasomy 12p). Hexasomy 12p was found in 20% of the cells by chromosome analysis of cultured skin fibroblasts and confirmed by FISH- and arrayCGH analysis. The girl has woolly and sparse hair, absence of lateral eyebrows, dry skin, brittle nails, hypopigmented patches, frontal bossing, hearing loss, hypertrophic dilated cardiomyopathy, polydactyly, and mild developmental delay. This is the second live case of mosaicism for hexasomy 12p to be reported. A gene-dosage hypothesis has previously suggested that cases with hexasomy 12p would have a worse phenotype than cases with tetrasomy 12p. The relatively mild symptoms found in the current girl with hexasomy 12p may to contradict this hypothesis. © 2009 Wiley-Liss, Inc.