The seronegative spondyloarthropathies appear to be the genetically predisposed host's clinical expression to acute, subacute or chronic reaction to the invasion by environmental microorganisms. In the ensuing days or weeks, depending on the infectious load, clinical manifestations may occur ranging from constitutional complaints such as fever, to a variety of symptoms and/or signs related to the portal of entry-intestinal, genitourinary or respiratory. Within weeks or months, the initial or other target organs, such as the mucocutaneous, ocular and cardiovascular systems, may develop an acute reaction of greater or lesser specificity regarding the triggering agent (oral ulcers, circinate balanitis, erythema nodosum, acute anterior uveitis, pericarditis, heart blocks). Lastly, many years later, a minority of patients, probably those with a large genetic component, exhibit a spectrum of clinical manifestations related to those organs, with a chronic or recurrent course. Acute clinical manifestations--reactive arthritis--are prominent in the initial phase of the clinical spectrum, while chronic manifestations--ankylosing spondylitis--are seen at the other end of the spectrum.