The use of flexible endoscopie sinus surgery (FESS) has undergone an exponential rise for the treatment of chronic sinusitis and polyposis. The early literature has demonstrated that a "learning curve" is operative in the reduction of FESS related complications. Some investigators have shown that the ever increasing number of procedures are associated with increasing complications. Major complications such as intranasal hemorrhage, CSF leaks, and orbital trauma have been previously described. We have reviewed five patients that we have treated and two others on which we have consulted in the last three years; this group includes five CSF leaks and two intracranial hemorrhages. No prior description of intracerebral or intraventricular hemorrhages have been reported. CSF leaks are reported to occur in approximately 0.18% of all FESS procedures. In all five patients with CSF leaks, diagnosis was established postoperatively and not recognized intraoperatively; 60% of patients had headache, 100% had rhinorrhea, and 40% had meningitis. Anosmia was complete in 2 of 5, and partial in 1 of 5. Diagnostic studies included positive tau transfer in 80%. Cisternography and MRI/CT characteristics are discussed. One leak resolved spontaneously and another with lumbar drainage. The other three persisted despite lumbar drainage. All had failed attempts at repair from below and required transcranial repair. Both patients with intracerebral hemorrhages required craniotomies and one required ventricular drainage for hydrocephalus. The characteristics of these injuries and CSF leaks are discussed as are the rationale and technique of operative repair. Recommendations for prevention,intraoperative recognition, and overall neurosurgical management of FESS related complications are presented.