From 10 October through 13 November 1989, 33 patients were hospitalized with legionnaires' disease in Bogalusa, Louisiana. A case-control study revealed case-patients were more likely than controls to have shopped at grocery store A (93% vs. 52%; odds ratio [OR], 11.6; 95% confidence interval [CI], 2.3-78.7) in the 10 days before illness. Among those who shopped at grocery store A, case-patients were more likely to shop for greater than 30 min (OR, 18.0; CI, 2.0-407.8) and to buy produce items located close to an ultrasonic mist machine (OR, 7.4; CI, 1.3-56.2). Employees of grocery store A were more likely than employees of other grocery stores in Bogalusa to have antibody titers of greater than or equal to 1:128 to Legionella pneumophila serogroup 1 (Lp-1; relative risk, 2.9; CI, 1.3-6.8). Lp-1 was isolated from water in the reservoir of the mist machine. The monoclonal antibody subtype of the isolate was identical to organisms identified in two patients. Viable Lp-1 was isolated from mist produced by the machine. Aerosols from a grocery store mist machine were the source of this outbreak.