Propionibacterium acnes is frequently referred to as a strictly anaerobic skin commensal of low pathogenicity, a description which is misleading. The bacterium is like Dr Jekyll and Mr Hyde, demonstrating both beneficial and harmful effects which are host- and possibly strain-dependent. Although the pilosebaceous follicles of the face and upper trunk are the major habitat of P. acnes, significant populations also exist in the healthy mouth and colon of a minority of individuals. P. acnes can behave as an opportunist pathogen in compromized hosts but the incidence of such infections is low or underestimated. There is considerable evidence that persistence of this phagocytosis-resistant bacterium in a variety of organs and tissues can lead to a chronic inflammatory response with granuloma formation. P. acnes has the capacity to produce enzymes which could contribute to tissue destruction in dental caries and periodontal disease. Whether some strains are more virulent than others or are associated with specific disease syndromes will be ascertained only when an adequate biotyping scheme has been developed. P. acnes should not be dismissed as a culture contaminant without good reason, especially in situations where other bacteria are not recovered. To date, attempts to harness the potent immunomodulating effects of the organism in cancer therapy have been disappointing.