Surveillance of 2,277 white-tailed deer for antibodies against Ehrlichia chaffeensis in Iowa showed seropositivity rates of 12.5% in 1994 and 13.9% in 1996. From 1994 to 1996, the estimated number of seropositive deer increased to 54,701 (28%). The increasing deer population and expanding tick distribution may increase risk for human monocytic ehrlichiosis.