Clear microscopic evidence of yeasts together with a detailed case history and results of the clinical examination are adequate for a definite diagnosis of vaginal mycoses. In principle, vaginal mycoses can only be ruled out by culture studies. This must be taken into account particularly when assessing the efficacy of antimycotics. Yeasts, in particular Candida albicans, are often detected on the mucosa and morphologically similar areas in man and animals without any symptoms being present. They are also occasionally found elsewhere, for example on fruit, foodstuffs, etc. This has led to the organisms being termed ‘facultative pathogenic saprophytes’ and to them being evaluated as relatively harmless. The term ‘facultative pathogenic opportunists’ is certainly more relevant. The problems in determining the pathogenicity of yeasts are similar to those encountered in the past with various bacteria. The pathogenic significance of yeast contamination is also linked to the interaction between organism- and host-related factors. In the case of organism-related factors, viable count and rate of proliferation are important as well as the different biochemical properties of the individual yeast strains, such as endo- and exotoxin formation and proteolytic activity. Endocrine disturbances, such as diabetes mellitus, hypothyroidism and hypoparathyroid-ism are accompanied by an increase in yeast infestation and a corresponding increase in the incidence of infection. The transition from yeast contamination to yeast infection largely depends upon the tolerance of the host or host tissue. This is lowered by a general reduction in resistance, antibody deficiency syndrome, avitaminosis, serum iron deficiency, etc. The well-known enhancing effect of various chemotherapeutic agents on the virulence of yeasts or responsiveness of the host organism is assessed in different ways; the threshold for the transition from contamination to infection is difficult to define, and the pathophysiological mechanism of yeast-related diseases is also unclear. Yeasts produce endo- and exotoxins only on a small scale, a purely toxic genesis is therefore improbable. Their interpretation as an allergic reaction, which is possible in gynaecological observations of acute mycotic vulvitis, is questionable since inflammatory reactions have also been observed in neonates with immature immunological systems during the first days of life and very occasionally even during intrauterine life. More information about the infectious process is expected from additional immunological and histochemical investigations. The following report deals with experimental investigations which provide an answer to some current p