The predominant form (65% of all patients) of yersiniosis is mesenteric adenitis, resulting in enteritis, pseudo-appendicular syndromes, ileitis or colitis. The severity of this form depends to some degree on the age. Extra-mesenteric forms (in 20-25%), focal infections by dissemination, the septic form and the lymphadenopathy syndrome may occur after or without previous enteritis. Persistence of Y. enterocolitica in lymphnodes and the GALT is responsible for chronic and recurrent forms. Secondary immunological complications (25%) are arthritis and Erythema nodosum. Diagnosis is made by culture, serology, of which demonstration of specific antibodies against released proteins is more reliable than demonstration of agglutinins and antigen detection by indirect immunofluorescence in biopsies. Antibiotic treatment of mild enteric forms is questionable, extra-mesenteric forms should be treated. Appropriate antibiotics are trimethoprim, chloramphenicol, tetracycline and the new fluoroquinolones. Studies are in progress (Leeuwarden and Hanover, MHH) to elucidate the indication of antibiotic treatment of Yersinia-arthritis patients.