A canine thyroid adenoma showing trisomy 18 as the sole clonal cytogen
etic abnormality (9 of 30 analyzed metaphases) is reported. Because tr
isomies are a recurrent cytogenetic finding in human benign thyroid tu
mors as well, it is suggested that the molecular relationship between
these trisomies and the development of thyroid tumors an be determined
by comparative gene mapping.