At a mixed patchy coniferous/deciduous forest in Brasschaat, Belgium, eddy correlation measurements of carbon dioxide flux were made above the forest canopy over a 5-yr period. The data were used to calculate the annual net ecosystem exchange. The CO sub(2) vertical flux was calculated as the covariance between fluctuations in the vertical wind speed and CO sub(2) concentrations. To estimate CO sub(2) storage in the air layer below the eddy correlation measurement height, measurements also included a profile of CO sub(2) concentrations at levels of 10, 24, 32, and 40 m above the ground. While net uptake fluxes were measured during the day, even in winter, net uptake on a daily basis only occurred from the beginning of April until the end of September, leading to a maximum length of the growing season of about six months. Net ecosystem exchange estimates were made using different gap filling methods, which showed that the uncertainty was relatively large. Overall, the forest was found to be a consistent net source of CO sub(2) of 110 g C/m super(2)/yr.