A 6-year-old girl with acquired immunodeficiency syndrome (AIDS) had extensive alveolar echinococcosis. Diagnosis was proved by needle biopsy of a specimen from affected liver tissue. The case was characterized by the unusually young age of the patient and more specifically by the absence of any detectable parasite-specific humoral and cellular immune response at diagnosis or later. Thus the present case indicates that a marked immunodeficiency caused by AIDS may substantially contribute to the rapid proliferation and early manifestation of the disease. In patients with pediatric AIDS and living in areas endemic for Echinococcus multilocularis, alveolar echinococcosis should be considered in intrahepatic tumor-like lesions even if the result of immunodiagnostic testing is negative.