During January-March 2006, a total of nine confirmed cases (based on onset date) of HPS were reported from Arizona, New Mexico, North Dakota, Texas, and Washington. Six of the nine cases were in Arizona and New Mexico. During 1994 and 1999, a similar number of HPS cases was confirmed nationally in the same 3-month period. Both years were characterized by environmental conditions (e.g., increased rainfall and vegetative biomass) during the preceding 1-2 years that promoted increased rodent populations. This, in turn, increased virus transmission in the rodent populations and increased exposure risk for humans.2-4 During 1994, 1999, and 2000, more than six cases were confirmed in the first 3 months of each year, and all had a high yearly total of HPS cases. Nine cases of HPS were identified in the first 3 months of 2006, suggesting that a greater risk for human hantavirus infection might exist this year.