Powdery mildew and downy mildew are among the most important diseases of grapevine. The relationship between these diseases and phenolic components has become increasingly important in recent years. This study evaluated changes of total phenolics, total antioxidant activity, and phenolic compounds (gallic acid and rutin hydrate) in some Vitis species. Vitis labrusca cultivars or genotypes, Vitis vinifera cultivars, interspecies hybrids, and a Vitis rotundifolia cultivar's healthy and infected leaves were examined. The contents of healthy material and of samples of plant material infected with both microorganisms (Uncinula necator (Schw.) Burr and Plasmopara viticola (Berk. & M. A. Curtis) Berl & De Toni) were determined. In general, the content of all compounds, except rutin hydrate, increased in infected leaves. After infection with powdery mildew, the increase was particularly significant. The total phenolic content rose after infection but varied with the cultivar and species. The highest antioxidant activity (598.96 mu M TE/100 g) and total phenolic levels (2248.27 mg GAE/100 g) were identified in V. vinifera 'Italia', which is known to be sensitive to fungal diseases. On the other hand, the highest levels of gallic acid (9.26 mg/100 g) were identified in V. rotundifolia 'Sugargate', which is known to be resistant to fungal diseases. The highest levels of rutin hydrate (60.44 mg/100 g) were identified in V. vinifera 'Red Globe'. There was a significant positive correlation between total phenolics and total antioxidant after both diseases (0.7556-0.6904).