To obtain data on infectious involvements contributing to joint attacks in Reiter's syndrome (RS) 72 consecutive males with RS were investigated. Controls were 219 personally interviewed males. A history of some type of oro-naso-pharyngeal or pulmonary infectious involvements were reported as often in the patients as in the controls, but the frequency of persons with a history of joint manifestations followed by these infections was significantly higher in the patients (23/72) than in the controls (8/219) (p less than 0.0001). Gastrointestinal involvements occurred only in a minority of cases as a triggering infection. Histories of prostatitis, gonorrhoea, condylomata acuminata, haematuria and trichomonas vaginalis infections were significantly more common in patients than in controls (p less than 0.001 - p less than 0.025). This finding, as well as evidence of chlamydial infection in 61%, speaks in favour of the importance of sexually transmitted diseases as an etiologic agent. A case report of triggering infections during the course of RS is given and the possible role of these infections in the immunopathogenesis in RS is discussed.