Introduction The IIHS is an international, prospective, multicenter study to compare endoscopic third ventriculostomy (ETV) and shunt in infants (<24 months old) with symptomatic triventricular hydrocephalus from aqueductal stensosis. Recruitment started in 2004, and here, we present the first results of IIHS. Methods IIHS utilized a prospective comprehensive cohort design, which contained both a randomized and a non-randomized arm. Patients received either an ETV or shunt, based on randomization or parental preference. Patients were followed prospectively for time to treatment failure, defined as the need for repeat CSF diversion procedure (shunt or ETV) or death due to hydrocephalus. Survival analysis was used to compare time to failure for ETV versus shunt. The trial was registered at clinicaltrials.gov (NCT00652470). Results A total of 158 patients met eligibility criteria (median age at surgery 3.6 months, IQR 1.6–6.6 months) across 27 centers in 4 continents. Since only 52 patients (32.9 %) were randomized, all 158 patients were analyzed together (115 ETV, 43 shunt). Actuarial success rates for ETV vs shunt at 3, 6, and 12 months were as follows: 68 vs 95 %, 66 vs 88 %, and 66 vs 83 %. The 6-month ETV success rate of 66 % was slightly higher than would have been predicted by the ETV Success Score (57 %).The hazard ratio for time to treatment failure favored shunt over ETV (3.17, 95 % CI 1.45–6.96, p  = 0.004), after adjusting for age at surgery, history of previous hemorrhage or infection, continent, and randomization status. Patients younger than 6 months of age appeared to do relatively worse with ETV than older patients. Conclusions The IIHS has provided the first prospective direct comparison of ETV and shunt for infant hydrocephalus. These initial results suggest that shunting has a superior success rate compared to ETV, although the success rate for both was relatively high. This patient cohort continues to be followed, and we will await the results of the important primary outcome of health status at 5 years of age.