Cytogenetic studies of one malignant and five benign human thyroid tumors grown in tissue culture are presented. In a follicular adenocarcinoma, a hyperdiploid stemline with supernumerary chromosomes in all groups was found. Abnormal chromosomes were not seen. In four of the adenomas studied, most of the cells had a normal chromosome complement. A history of prior x-irradiation to the face and neck was obtained from a fifty-four-year-old patient with a histologically benign microfollicular adenoma. Almost all of the cells had 46 chromosomes, but only a few had a normal karyo-type. The majority had a pseudodiploid chromosome complement due to either loss or acquisition of members of various groups, or the presence of abnormal chromosomes of varying size and configuration.