A randomised controlled trial was carried out to study the effect of 10 g of ascorbic acid taken during the first 2 1/2 days on the symptoms of the common cold. Altogether 1524 volunteers were recruited from a number of working groups in different parts of the country; 482 developed colds. There was no evidence that upper respiratory or general constitutional symptoms were alleviated by ascorbic acid. Among the men who had any colds at all, significantly fewer on active than on placebo treatment had two or more colds; however, this effect was not seen in women. Ascorbic acid is of no value in the treatment of the common cold; its preventive effect, if any, is not such as to justify advising its general use as a prophylactic measure.