The authors present the results of a review of 379 cases of benign and malignant thyroid tumors examined by high frequency (7 MHz) ultrasound prior to surgery. Features of the 37 cases of cancer are described. Hyperechogenicity was almost always correlated with benign lesions (only 1 cancer out of 71 hyperechoic nodules). For 313 of the 342 cases of benign tumors a good correlation was found between sonograms and intraoperative examination of the two thyroid lobes. In the case of clinically evident polyadenomatous goiters, ultrasound is a highly accurate means of determining whether any zones of healthy tissue remain (sensitivity 104/110 cases, i. e. 93.7%). When a contralateral lobe is normal on sonograms, intraoperative examination only rarely reveals lesions (12/159 cases, including 2 false positives for surgical investigation). Due to the excellent concordance between ultrasound and intraoperative exploration, and the difficulties involved in anterior dissection, the results of sonograms can be used to decide on the surgical approach. Direct intraoperative surgical exploration of the lobe opposite a thyroid lesion appears unnecessary if the sonogram is normal (cases of non suspected occult thyroid cancer). Since intraoperative exploration can give false negatives, intraoperative ultrasound can be used in the rare cases where 1 or 2 deep micronodules have been detected by pre-surgery sonograms in the lobe contralateral to the main lesion.