Fifty-seven patients with arthritis associated with non-gonococcal genital infection have been studied. Synovitis characteristically affected one or a few joints, expecially the knee, ankle or metatarsophalangeal joints and was accompanied by tenosynovitis and enthesopathies--each in about one third of the patients. A quarter of the patients had ocular, cutaneous, or mucous membrane lesions (Reiter's syndrome). Although six patients developed a chronic or relapsing course, average duration of the acute episode in the majority was three to five months. Available evidence strongly suggests that infection following sexual intercourse, usually but not always with a new partner, was instrumental in the initiation of the disease. We have suggested the term 'sexually acquired reactive arthritis (SARA)' to emphasize the mode of acquisition of the disease, and note that similar syndromes are seen associated with gut infection. We consider that usage of the term Reiter's syndrome is correctly applied to only those cases which exhibited the characteristic triad of urethritis, arthritis and conjunctivitis with or without other cutaneous and mucous membrane lesions. Thirty-six of the 54 patients who were HLA typed (67 per cent) possessed the antigen HLA-B27. Of 30 who presented directly to a rheumatology unit 25 (82 per cent) were HLA-B27 positive. The other 24 patients initially attended a venereology clinic and only 11 (46 per cent) of these bore the antigen. This appears to reflect disease severity, HLA-B27 positive patients having a significantly longer duration of disease symptoms and a higher frequency of extra-articular manifestations, than those lacking this antigen.