An idiopathic, asymmetrical oligoarthritis affecting young adults is prevalent in the tropical regions of Africa, Asia and Melanesia. A serological study was undertaken in 23 consecutive patients with polyarthritis. Each patient was assigned two paired control subjects. A standardized history, physical examination and investigations were performed. Acute and convalescent sera were examined for evidence of recent infection. The presence of HLA-B27 was determined. Twelve (52%) of the 23 patients had more than one of the clinical features of a reactive arthritis. No serological evidence was found of acute infection with viruses (arboviruses, enteroviruses, mumps, rubella, adenoviruses, Epstein-Barr virus and hepatitis B), Chlamydia, Mycoplasma, Yersinia or syphilis. HLA-B27 was found in 14 (61%) of the 23 patients in contrast with in seven (17%) of the 41 control subjects (P less than 0.001). In two patients with HLA-B27, diagnostic titres of antibodies to Campylobacter jejuni-Campylobacter coli indicated recent infection. The presence of HLA-B27 antigen was significantly (P less than 0.05) associated with the presence of the clinical features of reactive arthritis. This study suggests that "tropical arthritis" commonly is a reactive arthritis that is associated with the presence of HLA-B27.