We identified 31 patients with human immunodeficiency virus (HIV) infection and lung abscess. All patients had advanced HIV disease, and the mean CD4 cell count was 17/mm3 (range, 2-50/mm3). Twenty-two patients (71%) had previous opportunistic infections, and 24 (77%) had previous pulmonary infections. Symptoms at the time of presentation included fever (90% of patients), cough (87%), dyspnea (35%), pleuritic chest pain (26%), and hemoptysis (10%). The microbiological etiology was established for 28 patients, and the pathogens recovered were bacteria (65%), Pneumocystis carinii (6%), fungi (3%), and mixed microorganisms (16%). The pathogens included Pseudomonas aeruginosa (11), Streptococcus pneumoniae (6), P. carinii (5), Klebsiella pneumoniae (5), Staphylococcus aureus (4), Aspergillus species (3), viridans streptococcus (2), Haemophilus influenzae (1), Streptococcus milleri (1), Proteus mirabilis (1), and Cryptococcus neoformans (1). Mycobacterium tuberculosis was not isolated; two patients for whom a microbiological etiology was not established responded to antituberculous therapy. Patients were treated for 2-12 weeks; 25% of the patients received > 4 weeks of therapy. The outcome was poor: 36% of the patients had recurrences, and 19% died. In patients with AIDS, lung abscess is associated with advanced HIV infection, is due to a broad spectrum of pathogens, responds poorly to antibiotics, and has a poor prognosis.