The net exchange of CO2 (NEE) between a Scots pine (Pinus sylvestris L.) forest ecosystem in eastern Finland and the atmosphere was measured continuously by the eddy covariance (EC) technique over 4 years (1999–2002). The annual temperature coefficient (Q10) of ecosystem respiration (R) for these years, respectively, was 2.32, 2.66, 2.73 and 2.69. The light-saturated rate of photosynthesis (Amax) was highest in July or August, with an annual average Amax of 10.9, 14.6, 15.3 and 17.1 μmol m−2 s−1 in the 4 years, respectively. There was obvious seasonality in NEE, R and gross primary production (GPP), exhibiting a similar pattern to photosynthetically active radiation (PAR) and air temperature. The integrated daily NEE ranged from 2.59 to −4.97 g C m−2 day−1 in 1999, from 2.70 to −4.72 in 2000, from 2.61 to −4.71 in 2001 and from 5.27 to −4.88 in 2002. The maximum net C uptake occurred in July, with the exception of 2000, when it was in June. The interannual variation in ecosystem C flux was pronounced. The length of the growing season, based on net C uptake, was 179, 170, 175 and 176 days in 1999–2002, respectively, and annual net C sequestration was 152, 101, 172 and 205 g C m−2 yr−1. It is estimated that ecosystem respiration contributed 615, 591, 752 and 879 g C m−2 yr−1 to the NEE in these years, leading to an annual GPP of −768, −692, −924 and −1084 g C m−2 yr−1. It is concluded that temperature and PAR were the main determinants of the ecosystem CO2 flux. Interannual variations in net C sequestration are predominantly controlled by average air temperature and integrated radiation in spring and summer. Four years of EC data indicate that boreal Scots pine forest ecosystem in eastern Finland acts as a relatively powerful carbon sink. Carbon sequestration may benefit from warmer climatic conditions.