Anti-drug antibodies may develop with biological therapies, possibly leading to a reduction of treatment efficacy and to allergic and other adverse reactions. Patients with Gaucher disease were tested for anti-drug antibodies every 6 or 12 weeks in clinical studies of velaglucerase alfa enzyme replacement therapy, as part of a range of safety endpoints. In 10 studies between April 2004 and March 2015, 289 patients aged 2–84 years (median 43 years) were assessed for the development of anti-velaglucerase alfa antibodies. Sixty-four patients were treatment-naïve at baseline and 225 patients were switched to velaglucerase alfa from imiglucerase treatment. They received velaglucerase alfa treatment for a median of 36.4 weeks (interquartile range 26.4–155.4 weeks). Four patients (1.4%) became positive for anti-velaglucerase alfa IgG antibodies, two of whom had antibodies that were neutralizing in vitro, but there were no apparent changes in patients’ platelet counts, hemoglobin levels or levels of CCL18 and chitotriosidase, suggestive of clinical deterioration after anti-velaglucerase alfa antibodies were detected, and no infusion-related adverse events were reported. Less than 2% of patients exposed to velaglucerase alfa tested positive for antibodies and there was no apparent correlation between antivelaglucerase alfa antibodies and adverse events or pharmacodynamic or clinical responses.