Urban land area represents several percent of the conterminous U.S. and is rapidly growing. Vegetative cover averages 27% nationally for this landuse type and therefore may present an active and significant sink for co-incident anthropogenic emissions of carbon dioxide (CO2). Vegetation may also substantially increase evapotranspiration rates (especially in western cities) while lowering surface temperatures. Preliminary results will be reported of a study, begun June 2001, measuring trace gas, water vapor, and heat fluxes over a portion of the Denver urban area from a tall tower, using the eddy covariance method. Ancillary data includes carbon dioxide and water vapor concentration profiles (6 levels between 15-120m) and solar radiation. Under mid-summer, mid-day conditions, measurements indicated a net uptake of CO2 by the urban ecosystem.