In May 1993, an outbreak of hantavirus pulmonary syndrome (HPS) occurred in the southwestern United States. A case-control study determined risk factors for HPS. Seventeen case-patients were compared with 3 groups of controls: members of case-patient households (household controls), members of neighboring households (near controls), and members of randomly selected households > or = 24 km away (far controls). Investigators trapped more small rodents at case households than at near (P = .03) or far control households (P = .02). After the number of small rodents was controlled for, case-patients were more likely than household controls to hand plow (odds ratio [OR], 12.3; 95% confidence interval [CI], 1.1-143.0) or to clean feed storage areas (OR, 33.4; 95% CI, 1.7-666.0). Case-patients were more likely than near controls to plant (OR, 6.2; 95% CI, 1.1-34.0) and more likely than far controls to clean animal sheds (OR, 11.9; 95% CI, 1.4-103.0). Peridomestic cleaning, agricultural activities, and an increased number of small rodents at the household were associated with HPS.