Consumption of purchased leaf lettuce (i.e., red leaf, green leaf, bib, or romaine) was associated with an outbreak of E. coli O157:H7 infections in western Montana. The lettuces had been purchased from at least six grocery stores. These stores had received shipments from three distributors who had obtained their lettuce from two shippers. One shipper had received lettuce from a group of farms in Washington state; the other shipper was a small, local (Montana-based) produce grower. Epidemiologists suspected that the local producer was more likely to be the origin of this outbreak because the producer used compost (origen was cow manure) and because of the presence of animals near the water source for the lettuce. Also the lettuce from the Washington state farms was distributed more widely, and there was no evidence of outbreaks of this strain of E. coli O157:H7 in other locations.