In order to elucidate cytogenetic changes associated with the development of benign growth of follicular epithelial cells of the thyroid, cytogenetic analyses were performed on 340 follicular thyroid adenomas and goiters after short-term culture. Clonal chromosome changes were found in 67 cases. Trisomy 7 as the sole abnormality or along with other trisomies was the most frequent type of aberration (19 cases). Other recurrent numerical changes were loss of chromosome 22 (4 cases) and the second X or the Y chromosome (5 cases). Translocations involving 19q13 (12 cases) were frequent structural chromosome changes. After a histopathological classification of all cases, we have correlated the cytogenetic findings with the histology of the tumors. Only 8.4% of the goiters showed clonal abnormalities whereas 44.9% of the adenomas revealed clonal abnormalities. Furthermore, simple clonal changes were predominantly found in goiters and complex changes in adenomas. The most impressive correlation was found in the group of lesions with trisomy 7: Whereas all but one lesion with one or two additional trisomies were goiters, those having three or more additional trisomies were all adenomas or adenomatous goiters.