Since 1971, 21 placebo-controlled studies have been made to establish whether vitamin C at a dosage of > or = 1 g/day affects the common cold. These studies have not found any consistent evidence that vitamin C supplementation reduces the incidence of the common cold in the general population. Nevertheless, in each of the 21 studies, vitamin C reduced the duration of episodes and the severity of the symptoms of the common cold by an average of 23%. However, there have been large variations in the benefits observed, and clinical significance cannot be clearly inferred from the results. Still, the consistency of the results indicates that the role of vitamin C in the treatment of the common cold should be reconsidered.