An expression of ras oncogene protein (p21) was assessed with immunohistochemistry in normal, dysplasia, benign hyperplasia, chronic prostatitis and prostatic carcinomatous tissues. The results showed that the positivity rate of p21 was higher in prostatic carcinoma than that in other prostatic lesions and was markedly correlated to histologic tumor grade. The expression level of p21 was more intentive in prostatic dysplasia surrounding carcinoma than that in simple prostatic dysplasia. It is suggested that overexpression of ras oncogene p21 product may play an important role in progressing from dysplasia to cancer and may be used as a new tumor marker for assessment of biological behavior of prostatic carcinoma.