We conclude that chronic necrotizing pulmonary aspergillosis is a clinical entity which has not usually been recognized as one of the forms of pulmonary disease due to Aspergillus species. Patients are middle-aged, and often have some evidence of impairment of host defenses such as diabetes mellitus, a connective tissue disorder, poor nutrition, chronic obstructive lung disease or low dose corticosteroid therapy. They are almost always symptomatic with fever and a productive cough, and their chest roentgenogram shows infiltrative and cavitary disease, typical of a chronic destructive lung process such as tuberculosis or anaerobic infection. Cavity formation is often accompanied by the development of a mycetoma. The disease is usually of 1 to 6 months duration but can be present for years prior to diagnosis. The diagnosis is suggested by the clinical course and the isolation of the fungus from pulmonary secretions; negative cultures for other pathogens and failure to respond to antibacterial or antimycobacterial therapy are characteristic. The diagnosis is confirmed by pathologic evidence of tissue invasion by the fungus or a response to specific antimycotic therapy. The symptomatic response to antifungal chemotherapy, at times combined with surgical drainage or resection, is favorable. However, roentgenographic resolution is not uniform, and many patients have residual cavitary disease. The long-term prognosis is uncertain.