In mid-January 1993, an outbreak of Escherichia coli O157:H7 infections associated with eating hamburger patties at a fast-food restaurant chain (chain A) was reported in Washington State. From mid-December to mid-January, 9 cases of E coli O157:H7-associated bloody diarrhea and the hemolytic-uremic syndrome had been reported in San Diego County, California. A total of 34 persons had bloody diarrhea, the hemolytic-uremic syndrome, or E coli O157:H7 organisms isolated from stool during the period November 15, 1992, through January 31, 1993. Organisms of E coli O157:H7 identified from 6 persons were indistinguishable from those of the Washington outbreak strain. Illness was associated with eating at chain A restaurants in San Diego (odds ratio, 13; 95% confidence interval, 1.7, 99) and with eating regular-sized hamburgers (odds ratio, undefined; lower-limit 95% confidence interval, 1.3). Improved surveillance by mandating laboratory- and physician-based reporting of cases of E coli O157:H7 infection and the hemolytic-uremic syndrome might have alerted health officials to this outbreak sooner, which could have resulted in earlier investigation and the institution of measures to prevent more cases.