A microbiological survey has been undertaken of comedones isolated by micro-dissection from skin biopsies. Of closed comedones 10.7% and of open comedones 7.1% did not contain Pityrosporum spp., Propionibacterium spp. or Staphylococcus spp., the organisms most frequently associated with the pathogenesis of acne. Mature comedones were more frequently colonised than were young comedones. These results support the argument that the presence of microorganisms is not a prerequisite for comedo formation. Other pathological and ecological implications of these results are discussed.