There is scant information on the cell proliferation, apoptosis, oncogenes, and tumor suppressor genes status in adenosis. Forty-eight foci of adenosis were studied with immunohistochemistry for MIB-1; c-erbB-2, c-erbB-3, bcl-2 oncogenes; and p53. To evaluate apoptosis, the TdT dUTP nick end labeling (TUNEL) method was applied. Results were compared with the same studies on benign prostatic hyperplasia (BPH) (n = 20), low-grade prostatic intraepithelial neoplasia (PIN) (n = 10); high-grade PIN (n = 20), Gleason sum 2 to 6 cancer (n = 16); and Gleason sum 7 to 10 cancer (n = 22). MIB-1 proliferation index was lowest in BPH, followed by adenosis, low-grade prostatic intraepithelial neoplasia (PIN), low-grade cancer, high-grade PIN, and high-grade cancer. The apoptotic rate was generally low in all groups, although it was higher in PIN and cancer. In BPH and adenosis, bcl-2 was absent in luminal cells. In low- and high-grade PIN, both basal and luminal cells expressed bcl-2, whereas in cancer, expression was found in only 1 case (3%). C-erbB-2 showed absent or low values for cancer and adenosis, whereas it was commonly expressed in BPH and low- and high-grade PIN. Low expression in adenosis was also found with c-erbB-3 (6%) compared with all other groups. Expression of p53 was confined to cancer. Despite a significantly higher proliferation index rate compared with BPH, adenosis showed a markedly lower proliferating index when compared with low-grade PIN, high-grade PIN, and cancer. Expression of the oncogenes c-erbB-2 and cerbB-3 was very low in adenosis, and the staining pattern for bcl-2 was similar to that of BPH. These results provide additional evidence to that of prior studies that adenosis is a histological small acinar proliferation more akin to BPH than high-grade PIN or adenocarcinoma.