To identify the neurologic complications of critical medical illnesses, and to assess their effect on mortality rates and on medical ICU and hospital lengths of stay. Prospective clinical evaluation of all medical ICU admissions for 2 yrs. A 14-bed, general medical intensive and coronary care unit in a large university hospital. Patients (n = 1,850) admitted to the hospital, of whom 92 were admitted for primarily neurologic problems. Of the remaining 1,758 patients, 217 (12.3%) experienced a neurologic complication. None. Patients developing a neurologic complication while in the medical ICU demonstrated an increased risk of inhospital mortality when compared with patients who did not suffer such problems (45.7% vs. 26.6%; p < .00001). Patients with neurologic complications experienced 2.5-fold longer medical ICU stay times (p < .001) and almost two-fold longer hospital stay times (p < .001). Metabolic encephalopathy, seizures, hypoxic-ischemic encephalopathy, and stroke were the most common complications. Sepsis was the most frequent cause of encephalopathy, and cerebrovascular lesions were the most common cause of seizures. Formal neurologic consultations were requested in only 36% of these patients. Neurologic complications are associated with increased mortality rates and longer medical ICU and hospital lengths of stay. These conditions are probably underrecognized at present. ICUs have the potential to serve as environments for neurologic teaching and research.