Persistent infectious polyarthritis caused by Ureaplasma urealyticum in a patient with common variable hypogammaglobulinaemia is described. The patient developed a symmetrical, destructive polyarthritis and tenosynovitis associated with a markedly depressed synovial fluid glucose concentration and characteristic soft tissue abscesses. The ureaplasma organism developed resistance to multiple antibiotics and persisted for five years. The organism was identified repeatedly in many joints by culture, confirmed by DNA hybridisation, and mycoplasma-like structures were shown in synovial tissues by electron microscopy.