We examined whether the cell proliferation index by MIB-1, HER-2/neu gene amplification, Gleason grade, and pretreatment level of serum prostate specific antigen (PSA) correlated with postradiation recurrence (PRR) in patients with prostatic adenocarcinoma. Formalin-fixed, paraffin-embedded tissue sections from 42 pretreated cases of prostatic adenocarcinoma (38 needle biopsy and 4 transurethral resection specimens) were immunostained for MIB-1 (MMI, Ventana Medical Systems, Tucson, Ariz). HER-2/neu gene amplification was analyzed by fluorescence in situ hybridization using the Oncor unique sequence probe (Oncor, Gaithersburg, Md). The cell proliferation index by MIB-1 was determined by labeling index; levels of HER-2/neu were analyzed semiquantitatively. Twenty-three of 42 patients (55%) were considered to have PRR on the basis of consecutive elevations of serum levels of PSA to greater than 1.5 ng/mL after completion of treatment (mean follow-up time, 33.4 months). The cell proliferation index correlated with PRR on both univariate and multivariate analyses. Of the 23 tumors that showed PRR, 18 (78%) revealed a high cell proliferation index, compared with 6 of 19 cases (32%) that showed no PRR. Twelve of 23 cases of prostatic adenocarcinoma (52%) in the recurrent group showed HER-2/neu gene amplification, compared with 5 of 19 (26%) in the nonrecurrent group; these findings reached near significance on univariate analysis. Pretreatment levels of serum PSA also reached significance on multivariate analysis. In this preliminary study, the cell proliferation index by MIB-1 reached independent significance in predicting PRR in patients with prostatic adenocarcinoma, whereas HER-2/neu amplification by fluorescence in situ hybridization reached near significance.