We determined the diseases associated with extremely high levels of alkaline phosphatase in hospitalized patients. Computerized laboratory records of the Hospital of Saint Raphael identified all inpatients who had elevations of alkaline phosphatase above 1,000 U/l from April 1994 to September 1995. Thirty-seven inpatients with alkaline phosphatase levels above 1,000 U/l were identified. Six had bone involvement from malignancy or Paget's disease and were eliminated from further analysis, and 31 patients were included in the study. Levels of alkaline phosphatase ranged from 1,014 to 3,360 U/l. Ten patients had sepsis as the cause of the elevated alkaline phosphatase. These included gram-negative organisms, gram-positive organisms, and two patients with fungal sepsis. Seven of 10 patients with sepsis had an extremely high alkaline phosphatase level and a normal bilirubin, 3 of 10 patients with sepsis also had acquired immunodeficiency syndrome (AIDS). Eight patients had biliary obstruction, 7 with malignant obstruction and 1 with a common bile duct stone. Nine patients had AIDS. The cause of the elevated alkaline phosphatase in these included three with sepsis, three with mycobacterium avium intracellulare (MAI) infection, two with cytomegalovirus infection, and one with Dilantin toxicity. Three patients had diffuse liver metastases. Finally, four patients had benign intrahepatic disease, including one patient with liver hemangiomas, one patient with sarcoid hepatitis, one patient with lead toxicity, and one patient with drug-induced cholestasis. Extremely high elevations of alkaline phosphatase are most frequently seen in patients with sepsis, malignant obstruction, and AIDS. Patients with sepsis can have an extremely high alkaline phosphatase level and a normal bilirubin. A variety of other causes were also noted.