Three-year-old Scots pine (Pinus sylvestris L.) seedlings were grown for two years in the ground in open-top chambers supplied with either an ambient or elevated (ambient + 400 μmol mol−1) CO2 concentration. Phenological observations and measurements of height and stem diameter growth, absolute and relative growth rates, starch and soluble carbohydrate concentrations of the needles, and crown structure and needle properties were made at frequent intervals throughout the two growing seasons. Elevated CO2 significantly advanced the date of bud burst in both years. The increase in total needle area in response to elevated CO2 was accounted for by longer shoots and an increase in individual needle area in the first year, and by an increase in the number and length of shoots in the second year. Stem diameter and tree height were enhanced more by the elevated CO2 treatment in the first year than in the second, indicating a decreased effect of CO2 on growth over time. This was confirmed by a study of absolute and relative growth rates of leader shoots. During the first growing season of CO2 enrichment, mean weekly relative growth rates over the growing season (RGRm) were significantly enhanced. During the second year, RGRm in ambient CO2 closely matched that in elevated CO2.