There is currently no curative therapy for men who have disseminated prostate cancer following failed radical prostatectomy. The purpose of this trial was to investigate systemic radioimmunotherapy in these men. Eight patients with occult metastatic prostate cancer following radical prostatectomy as evidenced solely by a rising serum PSA and evidence of soft tissue lesions outside the prostatic fossa detected by an [111I]indiumcapromab pendetide scan received an infusion of 10 mg of capromab pendetide labeled with 9 mCi/m2 of [90Y]yttrium. Serum PSA was used to measure response rate. There were no complete or partial responses by PSA criteria. Significant unexpected bone marrow toxicity developed in the first 6 of 8 patients treated. The last two patients received co-infusion of edetate calcium disodium in an effort to decrease marrow suppression. In these two patients less marrow toxicity was seen. Repeat 111In-capromab pendetide scans were uninterpretable due to grossly altered whole-body biodistribution of the radioimmunoconjugate. Retrospective analysis of serial PSA values after closure of the study showed a decrease in the log slope PSA for seven of eight patients following radioimmunotherapy, with a statistically significant change in the mean log slope (p = 0.01). The clinical significance of this small but measurable change is uncertain. We conclude that radioimmunotherapy for occult metastatic prostate cancer using 90Y-capromab-pendetide at the dose described does not lower serum PSA, is associated with significant hematologic toxicity, and leads to complexation of the immunoconjugate following subsequent capromab pendetide infusion.