The role of 131Cs scan in the preoperative diagnosis of cancer was evaluated in 355 patients with either cold or nonfunctioning thyroid nodules. Nodules were classified as positive, doubtful or negative by the pattern of isotope accumulation. Among 234 patients who underwent thyroidectomy, malignant lesions were found in 10.2 per cent of cases. All carcinomas but one found during surgery had been classified as positive by radiocesium scan and were considered as highly suspicious preoperatively; one carcinoma and two papillary adenomas had been classified doubtful and considered presumably malignant. False-positive nodules were found. However, we did not document histologically malignant lesions in nodules which were classified as negative by radiocesium uptake. The routine use of 131Cs scanning may be very useful in patients with cold or nonfunctioning thyroid nodules because of its high sensitivity in excluding malignant lesions.