Propionibacterium acnes, a micro-organism with low pathogenicity, rarely causes arthritis in a joint without a prosthesis. This report deals with two cases of P acnes infectious arthritis probably caused by direct penetration of the micro-organism into the joint space during arthrocentesis, despite the use of strict disinfection procedures. P acnes isolated from synovial fluid cannot be dismissed as a bothersome contaminant without due consideration of the clinical status of the individual patient.