During 2006, a total of 4,261 cases of WNV disease in humans were reported from 731 counties in 43 states and the District of Columbia, accounting for 23.3% of the 3,142 counties in the United States. Of these cases, 1,491 were WNND, 2,612 were WNF, and 158 were unspecified illnesses. Idaho, a state that reported four WNND cases (from a total of 17 human cases) during 2003-2005, reported 139 WNND cases in 2006, accounting for 9.3% of the national total. Idaho first reported any WNV activity in 2002; the first human case in the state was reported in 2003. Other focal outbreaks of WNND occurred in states that experienced outbreaks in previous years, including Texas (229 WNND cases), Illinois (127), Louisiana (91), and Mississippi (89). In the New York City metropolitan area, WNV disease recurred for the eighth consecutive year, with eight WNND cases reported. The counties with the highest incidence of WNND were primarily in the west-central United States. The states with the highest incidence included Idaho (9.9 cases per 100,000 residents), South Dakota (4.9), and North Dakota (3.2). The incidence of WNND peaked during the first week in August, and the overall trend was consistent with the seasonality observed in the preceding 6 years.