Statistical survey on Japanese pathological autopsy cases revealed the incidence of deep fungus infections have been increased predominantly in recent years, and the vast majority of the infections in Japan were determined as opportunistic infections such as candidiasis and aspergillosis. Increased tendency of the cases with severe and multiferous patterns of the infections was pointed out. The incidence might be more increased with much attention to the infections and more careful study. The author's previous studies on various experimental fungus infections and diagnostic procedures such as the precipitin reaction using infected tissue extracts, fluorochrome stain, and fluorescent antibody technic were reviewed. Although an accurate histopathological pathogenetic genus diagnosis for the most of the deep fungus infections in Japan could be expected, the importance of the culture study and needs for the accumulations of precise knowledge on the tissue phase of fungi were also stressed. The morbid anatomical findings of candidiasis and aspergillosis were briefly reviewed. Special stress was layed on histological study on stainability, size, form, and microspectrophotometrical measurement of secondary fluorescence of fungus cells in tissue could be a reliable clue for the evaluation of the viability of fungal cells in infected tissue. The pathogenesis of opportunistic fungus infections was discussed from the basis of our investigations on fungal flora of the lung and the digestive canal and related studies, and concluded tha topportunistic fungas infection could be assumed as a terminal infectionof severely diseased, and the occurrence was commonly determined as a partial phenomenon of multiple infection with opportunistic bacteria, virus, and protozoa. Necessity of such understanding for the clinical diagnosis and treatment and pathological study was pointed out.