<h3>Background and aims</h3> Manifestations of Hunter syndrome typically become apparent between 2 and 4 years of age; affected children may be treated with enzyme replacement therapy with idursulfase (Shire). This long-term treatment consists of weekly infusions generally administered over 3 h. Patients may sometimes miss scheduled infusions. This analysis investigated the frequency of, and reasons for, missed idursulfase infusions and stopping treatment in children. <h3>Methods</h3> This analysis used data from the Hunter Outcome Survey (HOS), a global, observational registry sponsored by Shire that collects real-world clinical information on the natural history of Hunter syndrome and the long-term effectiveness and safety of idursulfase. <h3>Results</h3> As of January 2014, data on missed infusions and stopping treatment between HOS entry and last clinical evaluation recorded in HOS/treatment end (median, 35.4 months) were available for 483 children followed prospectively in HOS aged &lt; 12 years at initiation of idursulfase treatment. The mean time from treatment start to last evaluation/treatment end was 47.2 months. In total, 1046 missed infusions were reported in 135/483 children (28.0%). The most common reasons were illness (for 25.5% of missed infusions), holiday/vacation (10.0%) and caregiver/family issues (7.9%). At last evaluation, 31/483 patients (6.4%) had stopped treatment; the most common reason (38.7%) was the patient’s/parents’ decision. <h3>Conclusions</h3> Analysis of HOS data reveals that a variety of factors affect treatment compliance; the most common reason for missing an infusion was illness. However, 72.0% of children receiving idursulfase did not miss a single infusion during this analysis period, and few children stopped treatment.