Background. Twenty-seven ovarian cancer patients who failed chemotherapy entered a phase I/II trial of intraperitoneal 177Lu-CC49 antibody. Methods. Patients had disease confined to the abdominal cavity ± retroperitoneal lymph nodes, adequate organ function, and no previous radiation. Results. The most common side effects were delayed, transient arthralgia (10/27) and marrow suppression with 1.665 GBq/m2 (45 mCi/m2), which was considered the maximum tolerated dose. One of thirteen patients with gross disease had >50% tumor reduction after therapy, whereas most others with gross disease progressed (one went off study with stable disease at 11 weeks). Seven of nine patients with <1-cm nodules progressed in ≤21 months, and two of nine remain without evidence of disease at 4 to 5 months. Of patients with microscopic or occult disease, one relapsed at 10 months and four of five remain without evidence of disease at >6 to 35 months. Conclusions. Marrow suppression was the dose-limiting toxic effect of intraperitoneal immunotherapy with 177Lu-CC49. Antitumor effects were noted against chemotherapy-resistant ovarian cancer, even at lower dose levels, and resulted in prolonged disease-free survival of most patients with microscopic disease. This form of treatment deserves further study.