Background: Rituximab (RTX), a chimeric human-murine anti-CD20 monoclonal antibody, has been used for treatment of active moderate-severe Graves' orbitopathy (GO) since 2004 as second-line therapy in patients unresponsive to intravenous steroids. We conducted an open-label prospective study (EUDRACT 2012-001980-53) in which patients were treated with a single infusion of only 100 mg RTX to analyze the efficacy and safety of this low dose. Methods: Seventeen patients, of whom nine had disease that was unresponsive to intravenous methylprednisolone and eight with newly diagnosed GO, were enrolled. Disease activity was assessed with the clinical activity score (CAS) and severity with a composite ophthalmic score. Long-term surgical treatment and quality of life were also assessed, as well as treatment-related adverse events. Results: Mean baseline CAS was 4.56 ± 0.96 and decreased to 1.25 ± 1.14 at 24 weeks (p = 0.001). Disease inactivation occurred within 24 weeks in >90% of patients and was unrelated to disease duration. Severity improved in about 60% of patients, with no relapses. All patients showed peripheral depletion of CD20+ and CD19+ cells at the end of RTX infusion (60 minutes). Two patients required surgical orbital decompression because of optic neuropathy (ON). Among adverse events observed, there was one patient who developed a cytokine release syndrome. Conclusions: A dose of 100 mg RTX is effective in patients with active moderate-severe GO. Low doses are better tolerated, expose patients to immune suppression for a shorter period of time, and are extremely cost effective, compared with higher doses. This dose, consistently with all other immunosuppressants, does not prevent the progression of GO to dysthyroid ON.