The number of immunocompromised patients has increased during recent years. Most fungal infections in these patients are caused byCandida, Aspergillus, Mucor, andCryptococcusspecies. Patients with low granulocyte count are at the highest risk of invasive candidal infection. The commonest type of granulocytopenia is observed in connection with malignant diseases of the hematopoietic system. Cytotoxic treatment and radiotherapy of large-body areas tend to produce a significant decrease in circulating granulocytes. Early diagnosis and adequate treatment of fungal infections are mandatory for a successful outcome. In the oral cavity it is important to differentiate between colonization and invasive infection. The optimal approach to diagnosis is a combination of histology and cultivation of specimens obtained from the same site of suspected infection. Prophylaxis of oral fungal infection in immunocompromised patients is generally aimed at preventing colonization.