Abstract The opportunistic mould Aspergillus is the etiologic agent responsible for a variety of infections and conditions referred to as aspergillosis. These manifestations include allergic responses following exposure to the organisms (allergic bronchopulmonary aspergillosis), colonization with Aspergillus spp. (aspergilloma or fungus ball due to Aspergillus and other conditions such as external ear colonization) and invasive infection (invasive pulmonary aspergillosis and other clinical syndromes of tissue invasion). The importance of Aspergillus as a clinically important pathogen has increased dramatically in recent decades. Invasive aspergillosis is a significant cause of morbidity and mortality in high risk patients (Denning, 1998). A major factor associated with the increased number of cases of invasive aspergillosis is the increase in patients at risk for this disease, such as patients undergoing bone marrow or organ transplantation and patients with other immunodeficiencies. Unfortunately, definitive diagnosis of invasive aspergillosis remains difficult and is compounded by the fact that antifungal agents must be begun promptly if therapy is likely to be successful (von Eiff et al, 1995; Stevens et al, 2000a). However, as rapid diagnostic tools are not widely available to effectively establish a definite diagnosis of invasive aspergillosis, consideration of risk factors in specific epidemiological settings may be useful in suggesting a clinical diagnosis. In high-risk patients, clinical presentation as well as the use of alternative diagnostic procedures such as radiology may be useful in establishing a presumptive diagnosis of infection (Caillot et al, 1997; Patterson, 1998; Stevens et al, 2000a). Cultures may not always be positive in patients with invasive aspergillosis, but it is important to recognize that a positive culture result in high-risk patients may suggest the presence of infection (Patterson, 1999b). However, even when therapy is begun promptly, therapeutic outcomes with current agents remain poor, particularly in patients with disseminated disease (Denning, 1998; Patterson et al, 2000b; Stevens et al, 2000a). Recently, significant advances in the therapy of aspergillosis have been reported and therapeutic guidelines using currently available drugs have been published.