270 patients with a scintigraphically cold thyroid nodule of sonographically increased (n = 34), diminished (n = 72) or neutral (n = 86) echogenity or cystic criteria (n = 78) were subjected to fine needle aspiration biopsy. This revealed unequivocal malignancy in 8 and follicular neoplasia in another 30 patients, 10 of whom proved to have malignomas on further evaluation. A total of 12 papillary and 2 follicular carcinomas, 2 non-Hodgkin lymphomas, 1 sarcoma and the metastasis of a breast carcinoma were diagnosed. The most sensitive criteria for malignancy were diminished echogenity, an inhomogeneous echo pattern and the occurrence of a solitary nodule. The incidence of malignancy was increased among males but not among especially young persons. There was no sonographic feature that would permit omission of fine needle aspiration.