The authors measured alkaline phosphatase isozyme I (ALP-I) in sera of 24 brain-damaged patients and four with disorders other than brain damage. The study population comprised three patients with postresuscitation encephalopathy, four with ruptured cerebral aneurysms, 14 with acute subdural hematoma and cerebral contusion, and three with nontraumatic intracerebral hemorrhage. ALP-I detected in brain damage is physicochemically different from the other known ALP-Is that appear in patients with obstructive jaundice or hepatoma. In the brain-damaged patients, ALP-I became elevated about 7 days after admission and markedly increased as secondary brain damage developed. Excluding patients who died within 9 days of admission, the maximum serum ALP-I concentration was well correlated with the functional outcome. In cases in which barbiturate therapy was effective, the appearance of ALP-I was delayed and its elevation was suppressed. The results of this study suggest that measurement of serum ALP-I is useful not only in the management but also in predicting the prognosis of brain damage.