Identifying appropriate prostate cancer-associated antigens is critical to the development of immunologic treatments for prostate cancer. Flt3 ligand, a growth and differentiation factor for dendritic cells, has shown modest clinical activity in prostate cancer, presumably by eliciting anti-cancer immune responses. The authors previously reported the results of a clinical trial in which some patients with metastatic prostate cancer treated with flt3 ligand developed autoimmune hypothyroidism with thyroid-specific antibodies. Given these findings, the authors hypothesized that anti-prostate immune responses might also have been elicited in some subjects treated with flt3 ligand. The authors now report the identification of prostate-associated proteins immunologically recognized in 13 prostate cancer patients treated with multiple cycles of flt3 ligand. Using a normal prostate cDNA expression library, and sera from subjects before and after treatment with flt3 ligand, a modified SEREX approach was used to identify six proteins to which IgG immune responses were augmented after flt3 ligand treatment compared with pretreatment. IgG responses to one protein, MAD-CaP-5, were not detectable in any of the subjects' sera before treatment but were elicited after therapy in one subject. The authors suggest that MAD-CaP-5 could be explored as a novel prostate cancer antigen, and that this approach can be used to identify immunologically recognized proteins following any specific intervention.