Yersinia infections in 16 adults and 9 children are reported. 15 cases were caused by Yersinia pseudotuberculosis, 8 of them were proved by serological findings. In 4 cases the infection by Yersinia pseudotuberculosis was likely, in 3 other cases possible only. Real infections caused by serotype IV are demonstrated too. Furthermore there were 10 cases caused by Yersinia enterocolitica, 3 of them were proved by bacterial, the other ones by serological findings. Both germs caused identical symptoms: fever (80%), abdominal pains (56%), diarrhoea (52%), erythema nodosum (44%), arthritis (40%), vomiting (16%), weight loss (16%), lymphoma (12%) and others. In children 50% of erythema nodosum was produced by intestinal yersiniosis. The beginning with gastroenteritis and fever mostly was followed by a second phase with returning fever, abdominal pains, erythema nodosum and/or arthritis. Antibiotic therapy had a definite effect only in the first phase of gastroenteritis and in the two possibly relapsing cases. In two of 5 patients with long standing arthritis the HL-AB 27 was present.