The objective of this study was to define the severe neurological complications that occur in recipients of an orthotopic liver transplantation, receiving FK 506 as their primary immunosuppressive agent. To accomplish this, 100 consecutive orthotopic liver transplantation patients were followed prospectively from the time of their transplant until the date of their initial post-orthotopic liver transplantation discharge from hospital. All major neurological complications occurring during this period were recorded and assessed. The frequency of severe neurological complications occurring in these severely ill transplant recipients was 34%. Delirium was noted in 16, coma in 9, seizures in 4, and 5 developed focal motor deficits associated with the finding of a brain abscess, transient ischemic attack or central pontine myelinolysis. At the time at which a major neurologic complication was noted, the blood level of FK 506 was recorded. No direct relationship between FK 506 blood levels and the presence or absence of major neurologic complications of orthotopic liver transplantation could be demonstrated. Based upon this series, it can be concluded that although FK 506 may contribute to the pathogenesis of minor neurological complications seen after orthotopic liver transplantation such as tremors and headaches, the pathogenesis of most of the major neurologic complications occurring after orthotopic liver transplantation is multifactorial and cannot be ascribed solely to FK 506 toxicity.