Excessive cell proliferation and genetic changes such as loss of an allele (loss of heterozygosity (LOH)) or amplifications or deletions of parts of chromosomes (copy number variations (CNV)) are common findings in cancers. It is unknown whether these changes are also present in patients with overgrowth syndromes, although the presence of small-scale CNVs (such as duplication of 11p15 in Beckwith-Wiedemann syndrome), excessive cell proliferation and an increased frequency of tumors have all been reported in these patients. We present results of a genome-wide scan for LOH and CNV in Proteus syndrome (PS), a severely disfiguring overgrowth syndrome. We investigated CNV and LOH in DNA derived from affected and normal tissue samples from six PS patients using Affymetrix GeneChip Mapping 250 K Nsp high-density single-nucleotide polymorphism microarrays. Analysis revealed that LOH and CNVs were not common in PS. We attempted to validate selected CNVs detected by microarray analysis using quantitative genomic PCR, but the observed changes were not confirmed. These results suggest that large-scale genome-wide CNVs and LOH as seen in cancer syndromes are not characteristic findings in PS, although we cannot rule out the possibility that newer arrays with a higher number of probes could uncover smaller CNVs not detected in this study.