The utility of monoclonal antibody (MAb) imaging for detection of occult recurrent prostate cancer was investigated in 14 patients with elevated serum prostate-specific antigen at least 3 months after therapy. All were imaged with capromab pendetide (CYT-356) and subsequently had biopsies of the prostate bed. Ten also had PET scans with F-18 fluorodeoxyglucose. Ten MAb scans were positive for tumor in the prostate bed and eight showed lymph node metastases. Six of the seven patients with positive biopsies had positive MAb scans, one had a negative scan. Three of the seven patients with negative biopsies had negative MAb scans, four had positive scans. Of the six patients with positive biopsies who had PET scans, one was positive, five were negative. Two of four patients with negative biopsies had negative positron emission tomography scans, two were positive. MAb imaging is superior to PET scan for identifying recurrent disease in the prostate bed. Assuming no false-negative biopsies, the positive predictive values for MAb and PET scan are 60% and 33%, negative predictive values are 75% and 29% and sensitivities are 86% and 17%. Additional investigation is necessary to determine if MAb uptake in lymph nodes is predictive of metastatic disease.