We sought to estimate West Nile virus (WNV) activity in mosquito populations weekly at the census tract level in Chicago, IL, and to provide this information graphically. Each week we calculated a vector index (VI) for each mosquito trap then generated tract estimates using geographic information systems. During June 29–September 13, 2008, a median of 527 (60%) of 874 possible tracts per week had a VI value. Overall, 94% of the weekly VI tract estimates were 0; among those with a VI estimate greater than 0, the median was 0.33 (range 0.003–3.5). Officials deemed risk levels and weather conditions appropriate for adulticide treatments on 3 occasions, resulting in the treatment of approximately 252 linear kilometers of residential streets and alleys. Our analysis successfully converted complex, raw surveillance data into a format that highlighted areas of elevated WNV activity and facilitated the determination of appropriate response procedures.