Fungi of the genera Aspergillus, Candida, and Cryptococcus and the class Zygomycetes are the most common causes of thoracic opportunistic mycoses in immunocompromised patients. Candidiasis and zygomycosis usually manifest as severe, often life-threatening, pneumonias. Aspergillus species are commonly implicated as the causative organisms in a broad spectrum of pulmonary disorders, ranging from hypersensitivity lung disease in atopic patients to invasive pneumonia in immunocompromised patients. Cryptococcus neoformans infects both immunologically normal and abnormal patients, with variable clinical and radiologic findings. The diagnosis of an opportunistic mycosis requires familiarity with the epidemiology of the disease, the various modes of clinical presentation, and the full spectrum of radiologic manifestations. Because many of these fungi may normally colonize in the upper respiratory tract, sputum cultures are considered diagnostically unreliable. Instead, definitive diagnosis hinges on either culture of the fungus from infected tissue or demonstration of the organism at microscopic examination.