Twelve patients suffering from erythema nodosum who had diagnostic titres or seroconversions in chlamydial complement fixation test are described. Eleven had respiratory infections. The twelfth had sterile pyuria, symptoms of pelvic inflammatory disease and exceptionally high antibody titres against Chlamydia trachomatis in an immunofluorescence test. The simple chlamydial complement fixation test is recommended for the screening of cases of erythema nodosum.