The expression of p53 protein and proliferating cell nuclear antigen (PCNA) in 18 eccrine poromas and four porocarcinomas was examined by immunohistochemistry. Immunoreactivity for p53 in eccrine poromas was negative in five tumors, < 10% of tumor cells in one (low expresser), 10-50% in seven (moderate expressers), and > 50% in five (high expressers). The duration of the presence before excision of p53-negative poromas was shorter, and the size of these tumors was smaller in comparison with those of p53-positive poromas. Moreover, all high expressers showed some atypical cells in limited areas. Of the four porocarcinomas, three were high expressers and one a low expresser of p53 protein. The low-expresser tumor showed clinically more rapid growth and histologically no poromatous foci in contrast to the high expressers. No significant correlation was found between p53 protein expression and PCNA positive staining in either eccrine poromas or porocarcinomas. However, the percentages of PCNA-positive cells in porocarcinomas were significantly higher than those in poromas, with no overlapping values. These results suggest that the PCNA index is useful in differentiating between poroma and porocarcinoma and that p53 gene mutation may occur in long-standing eccrine poromas and correlate with atypical changes in histology as well as subsequent progression to porocarcinoma.