To analyze relevant factors of neoplastic transformation in oncocytic neoplasms of the thyroid, expression of p53, Ki-67, and bcl-2 has been studied in oncocytic carcinomas (n = 17) and compared with results obtained in oncocytic adenomas (n = 20). P53 protein accumulation was found immunohistochemically in 75% of the oncocytic adenomas (15 of 20) and 88% of the oncocytic carcinomas (15 of 17). Eight of 17 of the carcinomas (47%), but only 3 of the 20 adenomas (15%), showed nuclear p53 accumulation in more than 10% of the cells, mostly in a focal pattern. Ki-67 expression also differed significantly between adenomas and carcinomas. The median of Ki-67-positive cells was 12/10 high-power fields (HPF) for adenomas and 76/10 HPF for carcinomas (P < .001). Furthermore, metastatic carcinomas had a significantly higher Ki-67 positivity than nonmetastasized carcinomas (164/10 HPF v 42/10 HPF, P < .05). Bcl-2 immunohistochemistry showed a constantly positive reaction in normal thyroid tissue. In contrast, bcl-2 protein was not detected in most of the adenomas (70%) and carcinomas (76%). In conclusion, p53 protein and Ki-67 is more prevalent in oncocytic carcinomas than in oncocytic adenomas of the thyroid, indicating that these factors may be involved in the progression of oncocytic neoplasms in the thyroid. In contrast, loss of bcl-2 appears to be an early event in the formation of oncocytic neoplasms of the thyroid. Its importance for malignant transformation is, however, unclear.