A large scale double-blind trial was conducted to test the claim that the intake of one gram of vitamin C per day substantially reduces the frequency and duration of "colds". It was found that in terms of the average number of colds and days of sickness per subject the vitamin group experienced less illness than the placebo group, but the differences were smaller than have been claimed and were statistically not significant. However, there was a statistically significant difference (P <0.05) between the two groups in the number of subjects who remained free of illness throughout the study period. Furthermore the subjects receiving the vitamin experienced approximately 30% fewer total days of disability (confined to the house or off work) than those receiving the placebo, and this difference was statistically highly significant (P <0.001). The reduction in disability appeared to be due to a lower incidence of constitutional symptoms such as chills and severe malaise, and was seen in all types of acute illness, including those which did not involve the upper respiratory tract.