The opportunistic character of deep-seated mycoses depends on granulocyte-based defense in candidosis and aspergillosis. Therefore, haematological, patients represent the group of highest risk. Mucocutaneous candidosis is controlled by macrophages. Cryptococcus neoformans forces its way into the human host via causing an imbalance in the CD8-T-cell suppressor system. An aggravating synergism exists between Cryptococcus invasion and HIV-infection which explains the severe course of cryptococcosis in AIDS patients. The following pathways of transmission are observed in opportunistic mycoses: In aspergillosis and cryptococcosis humans are infected by inhalation of fungal propagules and primary settlement of the pathogens in the lungs, a site from which dissemination may occur. Exposure to Aspergillus conidiospores is ubiquitous and can be remarkably intensified by construction activities. Exposure to Cr. neoformans is geographically highly variable; not all humans are exposed. Candidosis emerges from the commensal reservoir of the human gastrointestinal tract, caused by translocation of the pathogens from the GI tract into the blood-lymph circulation or by anal/oral or oral-oral infection respectively. The incidence of deep-seated mycoses in the northern hemisphere is estimated to be 600 mycosis situations per million population per year. The estimate for Germany amounts to 50,000 mycoses patients per year, i.e. 45,000 candidosis and 5,000 aspergillosis situations as well as 100 each of cryptococcal and other mycotic diseases. Beyond that the number of AIDS-patients with cryptococcosis in Germany amounts to about 200 cases.