The role of the HER2 receptor remains uncertain in the pathogenesis and progression of human prostate cancer. Previous studies have reported widely divergent rates for HER2 expression in primary prostate tumors, probably owing to significant methodologic differences in the studies. Few data exist about the frequency of HER2 protein overexpression and gene amplification in androgen-independent prostate cancer (AIPC), although recent xenograft models suggest HER2 expression may be up-regulated in the transition from androgen-dependent to androgen-independent disease. We studied the role of HER2 protein in AIPC by immunohistochemical and fluorescence in situ hybridization (FISH) analyses on AIPC specimens using well-characterized and validated reagents. Fourteen (36%) of 39 specimens expressed HER2; however, only 2 (5%) had moderate (2+) expression, and 2 (5%) had high-level (3+) expression. Two (6%) of 36 specimens had gene amplification by FISH. These data suggest that HER2 protein overexpression and gene amplification are relatively uncommon in AIPC.