CONTEXT Shigella dysenteriae type 2 is rare in the United States, and outbreaks associated with this pathogen are uncommon. OBJECTIVE To determine the magnitude and source of an outbreak of S dysenteriae type 2. DESIGN Retrospective cohort. SETTING Laboratory of a large medical center. PATIENTS Case patients were identified as laboratory workers who had diarrhea on or after October 28 and a positive stool culture or temperature greater than 37.8 degrees C. Laboratory workers with diarrhea only were probable case patients. MAIN OUTCOME MEASURES We interviewed laboratory staff and performed identification, serotyping, and pulsed-field gel electrophoresis on isolates from case patients, implicated food, and laboratory stock culture. RESULTS From October 29 through November 1, a total of 12 (27%) of 45 laboratory staff developed severe, acute diarrheal illness; 8 had S dysenteriae isolated from stool and 4 were hospitalized. All case patients reported having eaten muffins or doughnuts placed in the staff break room on October 29. Pulsed-field gel electrophoresis showed stool isolates from 9 case patients were indistinguishable from S dysenteriae type 2 recovered from an uneaten muffin and from the laboratory's stock strain, a portion of which was missing. CONCLUSIONS The source of the outbreak was most likely the laboratory's stock culture, which was used to contaminate the pastries. Results of this investigation underscore the need for adequate precautions to prevent inadvertent or intentional contamination from highly pathogenic laboratory specimens.