Two years of continuous aerosol particle size distribution measurements provided the basis for this cloud droplet activation study. The cloud droplet activation of aerosol particles was studied in Pallas, a clean background site in northern Finland. A slightly different approach compared with traditional methods is presented by measuring simultaneously the cloud interstitial particle size spectrum and a nearby out-of-cloud particle size spectrum. The main advantage of this approach is that one can determine the activated fraction of different-size particles over the whole submicron-size range and that a large number of cloud activation events can be analyzed. The number of cloud droplet activation days peaked in late autumn and was lowest in summer. The annual variation of cloud droplet activation events followed the annual pattern of low clouds (those below 1000 m). A relation was found between the total particle number concentration outside the cloud and the number of activated particles. A larger particle concentration led to a higher number of activated particles, a lower activation percent, and a larger activation diameter (D50). D50 was on average 80 nm and varied from 50 to 128 nm. The average fraction of activated particles during the cloud events was 47% and varied from 9 to 86%. The cleaner and colder air masses from the northern Atlantic or the Arctic Ocean had, on average, 15 nm lower D50 than the more polluted air masses from south and east containing more particles. The annual variation of the number of the activated particles and other variables were also closely related to the annual variation of particle concentration (high in summer and low in winter). On average 87 and 30% of accumulation and Aitken mode particles, respectively, were activated. Aitken mode particles were observed to cover up to 55% of the total number of activated particles (i.e., number of formed droplets), which demonstrates that they have a significant effect on cloud droplet activation and must be taken into account when estimating the aerosol indirect climate effects.