During an 8.5-year period (1974 to 1982), 56 Yersinia enterocolitica isolates from patients residing mainly in the New York City area were studied. Evaluation of these isolates revealed a marked increase in the bacteremic episodes caused by strains of serogroups of O:3 and O:5,27 and the overall emergence of serogroup O:3, bioptype 4, phage type 9b Y. enterocolitica. Of the serogroup O:3 stool isolates, 59% (19/32) were recovered from patients in their first year of life. One of these subjects had concurrent serogroup O:3 bacteremia. The apparent establishment of serogroup O:3 Y. enterocolitica in urban communities raises the potential for widespread outbreaks of disease caused by these strains.