Current understanding of heat and water fluxes in urban environments derives largely from short-term, summertime, observation programs at a restricted number of sites (largely, though not exclusively, residential areas in North America). Urban observations of CO2 fluxes are even more limited, despite the fact that urban areas are known to be major sources of anthropogenic CO2. In May 2001, a new measurement program was initiated at a well-vegetated residential site in the Cub Hill area of Baltimore, USA. The objective is to measure heat, water and CO2 fluxes year round, and to investigate the effect of the urban ecosystem on carbon uptake/release rates. In this presentation, attention will focus on the seasonal differences in fluxes (heat, water and CO2) and the role of the residential vegetation. In the summer, strong daytime negative fluxes (uptake of CO2) are attributed to the strength of biospheric photosynthesis and strong mixing of local anthropogenic sources. As in other ecosystems, leaf-fall is associated with increasingly reduced rates of uptake and increasingly positive fluxes. The results will be discussed in the context of understanding from other ecosystems (largely from the AmeriFlux network) and a few very limited observations in urban environments.