A survey was carried out on a commercial sour cherry fruit orchard located in Lublin province in Poland to determine the influence of viruses on the fruit quality of sour cherry cv. Łutówka. Leaf samples from trees of sour cherry cv. Łutówka were tested for Prunus necrotic ringspot virus (PNRSV), Prune dwarf virus(PDV), Little cherry virus 1 (LChV-1), Little cherry virus 2 (LChV-2), Cherry virus A (CVA), Cherry green ring mottle virus (CGRMV), Cherry necrotic rusty mottle virus (CNRMV), Cherry rasp leaf virus (CRLV) and Cherry mottle leaf virus (CMLV) using the RT-PCR technique. The results indicated that PNRSV and PNRSV+CVA infected the samples. PDV, LChV-1, LChV-2, CGRMV, CNRMV, CRLV, and CMLV were not detected in any of the tested sour cherry trees. The effect of virus infection on the chemical composition of sour cherry fruits was investigated. The anthocyanin, total phenolic and vitamin C contents, and antioxidant activity were evaluated. The total phenolic compound, vitamin C contents, and antioxidant activity were significantly higher in PNRSV- and PNRSV+CVA-infected than in virus-free sour cherry fruits. The total anthocyanin content in PNRSV- or PNRSV+CVA-infected fruits was lower than in control trees. To our knowledge, this is the first report in the world about the effect of PNRSV or PNRSV+CVA infection on the anthocyanin compounds, total polyphenolic compounds, vitamin C contents, and the antioxidant activity of sour cherry fruits.