Purpose: We tested for a possible autoimmune process in benign prostatic hyperplasia (BPH). Materials and Methods: Titers of IgC antibodies to prostate specific antigen (PSA) were measured in the sera of 85 BPH patients, 20 controls and 17 chronic prostatitis patients by enzyme-linked immunosorbent assay. Results: The mean anti-PSA titers in the BPH group were significantly higher than in the controls and prostatitis group (p less than 0.0005). Accordingly, 59 percent of BPH patients could be defined as responders to PSA compared to none among the controls (p less than 0.0005). Conclusions: Circulating autoantibodies to PSA were shown to exist in the sera of BPH patients. This observation suggests that autoimmune processes may have a role in BPH.