In the Niagara Peninsula, cool years delay veraison, thus shortening the growing season and adversely affecting grape maturity. Exogenous abscisic acid (ABA) could potentially hasten veraison, and improve grape composition in cool and wet years. Two experiments were conducted in a Cabernet Sauvignon block in Niagara-on-the-Lake, Ontario, in 2008 and 2009. Both years were characterized by higher than average rainfall and lower temperatures. The first experiment was initiated one week preveraison and consisted of four treatments applied three times at two-week intervals: an untreated control and 300 mg ABA/L applied to the full canopy, clusters only, and leaves only. The second experiment had three treatments: 0 (control), 150, and 300 mg ABA/L applied to clusters only. In both years, the control still had clusters with 20% green berries two to four weeks after experiment initiation. Following treatment, berries had a lower ABA uptake rate than leaves. Both ABA rates hastened the onset of veraison. In both years the transpiration rate, leaf Ψ, and fruit composition were most affected in the leaves-only and whole canopy treatments. At harvest, Brix was higher and the berry weight was lower in the ABA treatments than the control. Total anthocyanins and total phenols also increased in most ABA treatments. Berries from clusters treated with the highest ABA rate showed a higher red-blue color intensity and also had highest anthocyanins and phenols content compared to berries from other treatments. The treated vines showed enhancement in individual anthocyanins and acetylated anthocyanins, with significant changes in the ratios of cyanidin, petunidin, and malvidin occurring among the treatments. Exogenous ABA was effective in accelerating onset of veraison and improving the grape composition of Cabernet Sauvignon.