This was a Phase I study for treatment with I-131 Metaiodobenzylguanidine (MIBG) in patients with metastatic carcinoid. The primary objective of the study was to determine the maximum tolerated dose of I-131 MIBG in patients with untreated metastatic carcinoid. There was documentation in the literature that patients with metastatic carcinoid, that uptake MIBG, can obtain at least subjective improvement in their disease related symptomatology with treatment using I-131 MIBG. The maximum tolerated dose had not yet been determined and that was the goal of the current study. We hoped that higher doses would improve response, both subjectively and objectively. Seventeen patients were enrolled in the study. Cohort #5 at 500 mCi of I-131 MIBG reached dose limiting toxicities with neutropenia (ANC<1500) and thrombocytopenia (<50,000) with a duration of 2 weeks or longer. These did not require hospitalization or transfusions, and no serious adverse reactions were seen. Their counts returned to normal by completion of follow-up (16 weeks). All patients showed subjective response and stable disease per CT scan. Only 3 patients received a second treatment on protocol. Three had progressive disease, 1 required surgery for tricuspid valve replacement and is stable, 14 were unable to afford further treatment due to the cost of the radioactive material or did not wish to pursue a second treatment at this time. The 3 patients that received a second treatment showed only subjective response with the first dose but did show an objective response with the second dose. No bone marrow toxicity was seen in this group of patients.