A population-based case-control study of gastric cancer was conducted in areas with contrasting incidence rates in Sweden. Face-to-face interviews were carried out with 338 (74.1%) of all eligible cases and 679 (77.3%) of the selected controls. Consumption of selected nutrients during adolescence and 20 years prior to interview was estimated, together with life-time intake of vitamin supplements. Ascorbic acid and beta-carotene had an unequivocal protective effect, but alpha-tocopherol and nitrate were also negatively associated with gastric-cancer risk. In a multivariate analysis including all of these factors, only ascorbic acid remained a significant protective factor. The only macronutrient positively associated with the risk of gastric cancer was fat: intake 20 years prior to interview, but not during adolescence, was found to have a significant impact. Supplementation with vitamins almost halved the risk after adjustment for dietary intake of the corresponding vitamins. While the protective effect of vitamin C and beta-carotene could conceivably be ascribed to other agents in the diet, the strong negative association between supplementation with vitamin C and risk of gastric cancer supports the hypothesis of a protective role of this anti-oxidant.