We investigated the relationship between the level of c-myc mRNA and histologic aggressiveness in thyroid tumors obtained at surgery. In thyroid carcinomas, there was a positive correlation between these two parameters, while in benign tumors there was no correlation between cellularity and the expression of the proto-oncogene c-myc. These results might be useful in the prognosis of thyroid tumors and consequently helpful in the management of the patient.