Human infections with Yersinia were first recognized and reported in upper New York State almost 40 years ago. Recently, there have been increasing numbers of reports from Scandinavia of episodes of polyarthritis associated with Yersinia infections. A 6-month-old girl from upstate New York had a fever of 40 C for two weeks and green watery diarrhea, and irritability was noted when she was handled. An evanescent rash was most apparent at times of temperature elevations. Her mother and father had had diarrhea during the month preceding the child's illness. Stool cultures were grown in an effort to identify Yersinia enterocolitica if it was present, and the organism was found. A rise in serum agglutination titer against polyvalent Y. enterocolitica was demonstrated from zero to 1:256 dilutions. Y. enterocolitica is probably a cause of arthritis, mesenteric adenitis, and erythema nodosum in the United States.