The aim of the study was to determine the properties of a standardized extract of ginseng root in inducing a higher immune response in vaccination against influenza. Attention was also paid to the common cold in this multicentre, two-arm, randomized, placebo-controlled, double-blind investigation. A total of 227 volunteers who visited 3 private practices in Milan received daily oral capsule doses of either placebo (113) or 100 mg of standardized ginseng extract Ginsana G 115 (114) for a period of 12 weeks within which they received an anti-influenza polyvalent vaccination at week 4. As a result, while the frequency of influenza or common cold between weeks 4 and 12 was 42 cases in the placebo group, it was only 15 cases in the G115 group, the difference being statistically highly significant (p < 0.001). Whereas antibody titres by week 8 rose to an average of 171 units in the placebo group, they rose to an average of 272 units in the G115 group (p < 0.0001). Natural killer (NK) activity levels at weeks 8 and 12 were nearly twice as high in the G115 group as compared to the placebo group (p < 0.0001). In all the volunteers, laboratory values of 24 safety parameters showed no significant differences between the end and the beginning of the 12-week study in either of the groups. There were only 9 adverse events in the study, the principal one being insomnia.