High hematological toxicity has been observed with anti-carcinoembryonic antigen radioimmunotherapy (RIT) in medullary thyroid carcinoma (MTC), suggesting metastatic bone involvement (BI). This retrospective study evaluated the rate of BI in MTC patients enrolled in two phase-I/II RIT trials using anti-carcinoembryonic antigen x anti-diethylenetriamine pentaacetic acid bispecific antibodies and [(131)I]di-diethylenetriamine pentaacetic acid hapten. Thirty-five patients underwent bone scintigraphy, bone magnetic resonance imaging (MRI), and post-RIT immunoscintigraphy (IS). IS performed in MTC patients was compared with IS conducted in 12 metastatic colorectal carcinoma (CRC) patients. Quantitative analysis of bone uptake was performed in three MTC and three CRC patients. In the MTC group, bone scintigraphy detected BI in 56.6% of patients, MRI in 75.8%, and IS in 88.6%. BI was confirmed by undirected (random) bone marrow biopsy, by bone surgery, or by two positive imaging methods in 74.3% of the patients. Sensitivity per patient of bone scintigraphy, MRI, and IS were 72.7, 100, and 100%, respectively. In contrast, IS visualized BI in only 33.3% of CRC patients; bone uptake was lower in CRC than in MTC patients. Bone MRI combined with post-RIT IS disclosed a much higher BI rate in advanced MTC than previously reported in the literature.