Pilomatricoma, also known as 'calcifying epithelioma of Malherbe', is a common skin adnexal tumor that mimics hair growth. Its proliferating cells seem distinctly programmed to undergo terminal differentiation and death. We report the first cytogenetic investigations of pilomatricoma. Trisomy 18 was shown, in an index case, by G-banded karyotyping. This aberration was corroborated by interphase fluorescence in situ hybridization, using a chromosome 18 pericentromeric probe, in the basaloid epithelial component of 7 of 11 pilomatricomas, including the index case. Trisomy 18 was present in a small subset of cells, suggesting a role in pilomatricoma progression, rather than in tumor initiation. We conclude that trisomy 18 is a consistent feature in pilomatricoma, suggesting that genes carried on this chromosome, such as that for the antiapoptotic oncoprotein BCL2, may have a role in the growth and differentiation of this benign self-limited tumor.