A protracted, outbreak of E. coli O111:NM occurred at the Four Mile Prison, a Colorado correctional facility. The correctional facility housed its own dairy. During April–July, 10 inmates at the facility were diagnosed with E. coli O111(STEC O111) infection. A retrospective prevalence study of 100 inmates found that, during March–April, 14 other inmates had diarrheal illness suspected of being STEC O111 infection. Pulsed-field gel electrophoresis (PFGE) testing indicated that STEC O111 isolates from inmates matched STEC O111 isolates from cattle manure at the onsite dairy. Several dairy workers had symptoms consistent with E. coli O111 infection. The environmental investigation found opportunities for contamination of the correctional facility by dairy workers.