The yeast Saccharomyces cerevisiae is used widely used in baking, brewing, wine making, and biotechnology and previously has had GRAS (generally regarded as safe) status. Recent evidence indicates the involvement of S. cerevisiae in a range of superficial and systemic diseases. Numerous cases of S. cerevisiae-induced vaginitis have been documented as have cases of oropharyngeal infection. Potentially fatal systemic disease due to S. cerevisiae has been recorded in bone marrow transplant patients and in those immunocompromised as a result of cancer or AIDS. A number of studies have indicated that commercially available strains of S. cerevisiae may cause disease in certain individuals. Pathogenic isolates exhibit the ability to grow at 42 degrees C, produce proteinase, and are capable of pseudohyphal growth. In addition, a number of isolates are capable of phenotypic switching and show partial or complete resistance to commonly used antifungal agents, including fluconazole. In the light of these findings, S. cerevisiae should now be regarded as an opportunistic pathogen, albeit of relatively low virulence, and treated accordingly by those in the industrial and biotechnological sectors.