Background. Nucleolar organizer regions (NORs) are loops of ribosomal DNA that occur in nucleoli and that transcribe to ribosomal RNA. NORs have been identified by means of the Ag‐NOR technique in routinely processed tissues, and were found to be of discriminative value between some types of benign and malignant lesions. Methods. Follicular lesions of the thyroid (17 adenomas and 25 carcinomas) were examined. Ten normal thyroids served as the control group. All slides were stained by the Ag‐NOR technique and the number of Ag‐NOR dots were counted in 50 randomly selected cells. The mean number of Ag‐NORs was calculated for each case. Data were statistically analyzed by the Student's unpaired t test. Results. The mean Ag‐NOR counts were statistically higher in follicular carcinomas as compared to either follicular adenomas or the normal thyroid. Higher Ag‐NOR counts were found in the more aggressively behaving tumors. Conclusions. It is suggested that the Ag‐NOR technique could be of use as an adjunct to diagnostic histopathology and as an indicator of biologic behavior in follicular tumors of the thyroid. © 1994 John Wiley & Sons, Inc.