A child whose features are consistent with Pallister-Killian syndrome (PKS) did not have detectable tetrasomy 12p due to an additional isochromosome 12p in an unstimulated blood specimen by interphase FISH or array CGH analysis. The diagnosis of PKS was made through these methods solely in a skin biopsy specimen. To determine the sensitivity of our array CGH platform to tetrasomy 12p mosaicism, dilutions of DNA from both the child's skin fibroblasts and a PKS cell line were analyzed. Tetrasomy 12p at 10% mosaicism was identifiable but 5% was below the limit of detection. This result suggests through extrapolation that the tetrasomy 12p is present in <10% of cells in our patient's blood, confirming the tissue-limited mosaicism of PKS. Multiple recent studies show that array CGH provides greater sensitivity than chromosome analysis to detect mosaic abnormalities including that of tetrasomy 12p in blood specimens. However, our case demonstrates that the biology of PKS precludes the exclusive use of array CGH on blood for diagnosis. A tissue sample should continue to be the diagnostic gold standard for PKS.