Phytochemicals may protect cellular DNA by direct antioxidant effect or modulation of the DNA repair activity. We investigated the repair activity towards oxidised DNA in human mononuclear blood cells (MNBC) in two placebo-controlled antioxidant intervention studies as follows: (1) well-nourished subjects who ingested 600 g fruits and vegetables, or tablets containing the equivalent amount of vitamins and minerals, for 24 d; (2) poorly nourished male smokers who ingested 500 mg vitamin C/d as slow- or plain-release formulations together with 182 mg vitamin E/d for 4 weeks. The mean baseline levels of DNA repair incisions were 65·2 (95 % CI 60·4, 70·0) and 86·1 (95 % CI 76·2, 99·9) among the male smokers and well-nourished subjects, respectively. The male smokers also had high baseline levels of oxidised guanines in MNBC. After supplementation, only the male smokers supplemented with slow-release vitamin C tablets had increased DNA repair activity (27 (95 % CI 12, 41) % higher incision activity). These subjects also benefited from the supplementation by reduced levels of oxidised guanines in MNBC. In conclusion, nutritional status, DNA repair activity and DNA damage are linked, and beneficial effects of antioxidants might only be observed among poorly nourished subjects with high levels of oxidised DNA damage and low repair activity.