Polymerase chain reaction (PCR) amplification, which is a useful method for detecting infectious agents in joints, has potential utility in the molecular diagnosis of venereal-associated arthritis. Among pathogens detected by this technique, Ureuplusrnu ureulyticum, which is primarily associated with reactive arthritis (ReA), is also implicated in septic arthritis in immunocompromised patients. We report here a case of destructive polyarthritis, initially suggestive of septic arthritis, in an immunocompetent patient whose PCR positivity for U ureulyticum DNA in one joint, in conjunction with the disease outcome and histologic findings, led to the diagnosis of destructive ReA.Mycoplasmal organisms (mycoplasmas and ureaplasmas), which have a predilection for mucous membrancs, are responsible for a broad spectrum of clinical diseases. In immunocompetent individuals, the respiratory (Mycoplasma pneumoniae) and urogenital (Mycoplasma hominis, Ureaplasma urealyticum) tracts are primarily involved. In immunocompromised patients, these microorganisms are able to disseminate to other anatomic sites, particularly the joints, and some cases of septic arthritis due to U urealyticum have been described (1-7). These infectious agents have also been implicated in the pathogenesis of reactive arthritis (ReA) (8) and, possibly, rheumatoid arthritis (9). These arthritides should be considered the result of an immunologic ~~