We studied the interannual variability of cumulative net ecosystem CO2 exchange (NEE) and its connection with cumulative or average climatic variables during five growing seasons. The analysis was based on a 5-year-long time series of CO2 flux measured from April 1996 to April 2001 in a Scots pine forest in southern Finland by the eddy covariance technique. The onset of the ecosystem growing season was best connected with air temperature, and the end of the growing season was best connected with day length. With these variables we were able to predict the timing and the length of each growing season within 0--3 days. The forest was a sink of carbon with little interannual variability: The uptake during the four full growing seasons varied by 80 g C m-2, ranging from 230 to 310 g C m-2. The estimated winter release each year varied between 60 and 90 g C m-2. The interannual variation in seasonal (spring, summer, autumn) carbon exchange ranged from 30 g C m-2 in autumn and spring to 80 g C m-2 in summer. The average climatic variables explained the variability of the seasonal or growing-season cumulative NEE only partly. Both the daytime and the nighttime CO2 fluxes contributed markedly to the variability in carbon exchange, indicating that photosynthesis and respiration have an equally important influence on NEE.