Mucopolysaccharidosis type II (MPS II; Hunter syndrome; OMIM 309900) is a life-limiting, multisystemic disease with varying presentation and severity. Enzyme replacement therapy with intravenous idursulfase (EC 126.96.36.199) has been available since 2006. Data from the Hunter Outcome Survey (July 2016) were used to compare survival in idursulfase-treated (n = 800) and untreated (n = 95) male patients followed prospectively in this multinational, observational registry. Median age at symptom onset was similar for the treated and untreated groups (1.6 and 1.5 years, respectively), as was median age at diagnosis (3.3 and 3.2 years) and the proportion of patients with cognitive impairment (58.0%; 57.9%). The proportion of idursulfase-treated patients differed according to geographical region. Overall, 124/800 (15.5%) treated and 28/95 (29.5%) untreated patients had died. Respiratory failure was the most common cause of death (treated, 43/124 [34.7%]; untreated, 10/28 [35.7%]). Median survival (95% confidence interval [CI]) was 33.0 (30.4, 38.4) years in treated patients and 21.2 (16.1, 31.5) years in untreated patients; median follow-up time from birth to death or last visit was 13.0 and 15.1 years, respectively. A Cox model adjusted for treatment status, cognitive impairment, region and age at diagnosis indicated a 54% lower risk of death in treated compared with untreated patients: hazard ratio (HR), 0.46 (95% CI: 0.29, 0.72). Patients with cognitive impairment had nearly a fivefold higher risk of death than those without (HR, 4.84 [3.13, 7.47]). This analysis in a large population of patients with MPS II indicates for the first time that idursulfase treatment is associated with increased survival.