Nineteen patients were involved in an outbreak of infection caused by Yersinia pseudotuberculosis serotype 3. No epidemics attributable to this microorganism have been previously reported; the most extensive known cluster of cases involved four children in one family and their pet dog. The key finding in the outbreak described in the present study was the bacteriologic identification of serotype 3 in stool specimens from patients with clinically typical yersiniosis. Twelve cases were identified by isolation of Y pseudotuberculosis from stool specimens. An ELISA permitted serological diagnosis of the remaining seven cases. The antibody response was unusually slow in some patients. A noteworthy feature of the outbreak was the high incidence of postinfection complications, which developed in 10 of 19 patients. In spite of active screening of the respective families and environments of the patients, no transmitting factor was found, and the precise source of the infection remains unknown.