So far over 60 studies have examined the effects of vitamin C on the common cold. No effect on common cold incidence was observed in the six largest studies, indicating that vitamin C has no preventive effects in normally nourished subjects in the Western countries. There are, however, smaller studies reporting benefit. In three trials of subjects under heavy acute physical stress, common cold incidence decreased by on average 50%, and in four trials of British males common cold incidence decreased by on average 30% in the vitamin C groups. The dietary vitamin C intake in the UK is low, and consequently the benefit may be due to the correction of marginal deficiency, rather than high vitamin doses. Regular vitamin C supplementation (> or =1 g/day) has quite consistently reduced the duration of colds, but the size of the benefit has varied greatly. In the four largest studies the duration of colds was reduced only by 5%. In two of these studies, however, absence from school and work was reduced by 14-21% per episode, which may have practical importance. Three controlled studies recorded a reduction of at least 80% in the incidence of pneumonia in the vitamin C group, and one randomised trial reported substantial treatment benefit from vitamin C in elderly UK patients hospitalized with pneumonia or bronchitis. It seems that the preventive effects of supplementation are mainly limited to subjects with low dietary vitamin C intake, but therapeutic effects may occur in wider population groups. Further carefully designed trials are needed to explore the effects of vitamin C.