Yersinia enterocolitica (Y. ent.) infections are rather frequently complicated by acute reactive inflammation in the connective tissue, especially in the joints. At this stage of the disease the specific diagnosis can be obtained either by bacterial isolation and identification from the feces and/or mesenterial lymph nodes, or by serological methods. Serodiagnostics are frequently the only method during the complication phase, since the bacteria have often disappeared from the feces by this stage of the disease. Specific Y. ent. Serodiagnostics are benefitted by the fact that no antisera cross-react with the serotype 3 thermostable O-antigen. A titre of≥80 is therefore highly indicative of a recent or current Y. ent. infection. In the absence of other known arthritogenic agents the Y. ent. antibodies are highly indicative of the Y. ent. etiology of a current disease. The Y. ent. complications affect most inflammatory reactive diseases, acute as well as chronic. In an area in which Y. ent. infections are endemic, Y. ent. is the most frequent cause of acute and chronic arthritis. The present results indicate that not all cases of acute Y. ent. arthritis remit, but some persist, usually with an intermittent course, and develop into rheumatoid arthritis or allied conditions. This suggests a common pathogenic mechanism in most inflammatory rheumatic diseases. It is proposed that the time has come for a classification of these diseases based on their etiology, in order to replace the present symptom-based treatment with a causal one, and to institute prophylactic measures. The pathology is not exclusive to Y. ent., but can presumably also be brought about by other bacteria, such as gonococci, meningococci, salmonellae, shigellae, and brucellae, possibly by their content of lipopolysaccharide.