Résumé : A patient with the Acquired ImmunoDeficiency Syndrome (AIDS) treated with a daily low dose of corticosteroids for chronic atopic dermatitis experienced a sudden episode of unilateral knee arthritis. Culture of the purulent synovial liquid yielded a pure culture of Streptococcus Equisimilis. A four week period of intravenous antibiotherapy combined with repeated drainages allowed a complete recovery of articular function.