A variety of gross, histological and cytological features were analyzed in a comparative study between adenomas and carcinomas of the parathyroid gland. The histological findings more valuable in the differential diagnosis of parathyroid carcinoma were the presence of a thick and irregular capsule infiltrated by the tumor (Fig. 1), broad and irregular intratumoral fibrous septa, and capsular or intratumoral vascular invasion (Table 1). Cytologically, the carcinomas were composed only by chief cells, with a mitotic activity higher than in adenomas, showing the presence of abnormal mitosis (Table 2, Fig 4). Characteristically all carcinomas were composed by a cellular population significantly of greater size than the one observed in adenomas.