Among 34 children with AIDS enrolled in a trial with 2'-3' dideoxyinosine (ddI), six (aged 1-6 years) developed liver abnormalities within 2-18 months after institution of ddI. Two children died of fulminant hepatic failure (one had also an adenovirus infection), and four had a striking elevation of alkaline phosphatases (AP: 1120-7000 IU/L). All of them received sulfa drugs and antifungic treatment with ketoconazole or fluconazole. Three had a serology positive for hepatitis C virus. The evolution of liver enzymes following withdrawal and rechallenge with ddI in the children with elevated AP was an indication of drug-induced toxicity in two patients. In both of the others, readministration of ddI did not cause any subsequent problems. Liver histology in the patients with fulminant hepatitis showed an extensive hepatic necrosis. Liver biopsy done in two other patients revealed a mild granulomatous hepatitis in one and nonspecific changes (i.e., steatosis and a mild inflammation) in the other. Hepatic toxicity has been described with ddI and other nucleoside analogs in adult patients. These six children had many potential causes of liver disease. It may be that ddI is not hepatotoxic per se but that it precipitates liver disease in predisposed patients. We recommend that liver functions be carefully monitored when using this drug.