Reactive arthritis (ReA) is a seronegative oligoarthritis triggered by a preceding extra-articular infection. While evidence of a microbial infection is mandatory for establishing the diagnosis of ReA, the sensitivity of bacteriological and serological tests has not been determined in patients without symptoms of infection. In a retrospective study, we evaluated the usefulness of urogenital swab cultures, serology and stool culture to identify infections in 234 patients with undifferentiated oligoarthritis. One hundred and forty-four patients complaining about joint pain who had no sign or history of inflammatory arthritis served as controls. Urogenital swab cultures showed a microbial infection in 44% of the patients with oligoarthritis (15% Chlamydia, 14% Mycoplasma, 28% Ureaplasma), whereas in the control group only 26% had a positive result (4% Chlamydia, 7% Mycoplasma, 21% Ureaplasma) (P < 0.001). A Chlamydia IgG-antibody titre > or = 1:256 was found in 22% of the patients in the oligoarthritis group and in 9% of the controls (P < 0.01). However, for only half of Chlamydia IgG-positive patients could a Chlamydia infection be confirmed by urogenital swab culture. Twenty-one per cent of patients with oligoarthritis vs 23% of the controls had positive antibody titres for Salmonella (not significant), 15% vs 5% for Yersinia (P < 0.05) and 17% vs 3% for Borrelia IgG (P < 0.01). In two patients, stool cultures were positive for Campylobacter. Urogenital swab culture is a sensitive diagnostic method to identify the triggering infection in ReA. A single determination of antibodies against Chlamydia trachomatis is of limited value because of the high prevalence of positive results in the control group.