OBJECT: The rationale for using endoscopy to treat complex cysts and multiloculated hydrocephalus is to combine several CSF compartments into a minimum number, establish a connection to functioning CSF compartments (that is, ventricles), and decrease shunt dependency. The aim is to decrease the number of proximal shunt catheters, the number of shunt revisions, and in selected cases even to avoid a shunt. In cases of distorted anatomy and multiloculated cysts, endoscopy may be problematic because of orientation issues. Standard navigation becomes useless soon after CSF loss due to brain shift. Therefore, the concept of "real-time" navigation and intraoperative imaging in combination with endoscopic surgery has been previously suggested. The goal of the present study was to assess the feasibility and efficacy of combining intraoperative MR (iMR) imaging and navigated neuroendoscopy in infants. METHODS: The authors report their experience in treating 5 infants (aged 6-14 months), who underwent surgery for multicystic hydrocephalus presenting with shunt malfunction (4 patients) and a quadrigeminal fetal arachnoid cyst (1 patient). In all infants, a low-field portable iMR imaging system (0.12-T PoleStar N-10/0.15-Tesla PoleStar N-20) was used in conjunction with navigated endoscopy. The authors used e-steady, T1-weighted, and T2-weighted sequences (acquisition time 24 seconds to 3.5 minutes). RESULTS: The iMR imaging system provided clear images that correlated with the endoscopic appearance of the cystic membranes in all patients, and the images were helpful in determining trajectories and redefining targets. The iMR images documented brain shift and changes in CSF spaces during surgery. There were no intraoperative complications or technical difficulties of visualization. No infection or any other immediate postoperative complication occurred. Patients were followed up for 9 months to 7 years. The infant presenting with the quadrigeminal cyst remains shunt free since surgery, and the patients with multicystic hydrocephalus have 1-2 shunts each. Following endoscopic, iMR imaging-guided surgery, shunt catheter positioning was found to be optimal and as planned according to the postoperative imaging. CONCLUSIONS: Navigated neuroendoscopy and iMR imaging may complement each other, offering an advantage over other modalities in complicated cases of hydrocephalus. Whenever targets and trajectories need to be redefined, the iMR images provided an updated navigation data set, allowing accurate navigation of the endoscope and minimizing the number of CSF compartments. Direct vision through the endoscope provides microanatomical details for the optimization of fenestration and catheter positioning. The combined usage of the two modalities may transform a conventional procedure into a visually controlled real-time navigated process.