We analyzed the effects of an accidental tenfold overdose of influenza vaccine that was given to 31 young healthy members of the Israel Defense Forces. A clinical workup and serological tests were performed after the vaccination in order to quantify any side effects or complications and relate them to immunogenicity of the vaccine administered. A small age-matched control group of 20 soldiers of the Israel Defense Forces who received normal doses (0.5 ml) of vaccine was subjected to the same examinations. A low vaccine efficacy was noted with the recommended 0.5-ml dose (7 micrograms of hemagglutinin of each influenza type), shown by low fourfold titer rises. A significantly higher number of fourfold changes was noted, as expected, in the study group that received 5.0 ml. Pre- and postvaccination geometric mean titers (GMT) in the control group for A/Bangkok (H3N2), A/Brazil (H1N1) and B/Singapore rose from 21.42 to 47.56, from 13.89 to 45.74 and from 10.44 to 11.68, respectively. In the study group the elevation was significantly more pronounced, GMT rising from 19.15 to 83.54, from 16.81 to 96.60 and from 10.00 to 18.88, respectively. Some minor reactions, such as headache and general malaise, were observed in both the study and control groups, but were more frequent in the former. No severe immediate or long-term local and systemic side effects were noted in either group. The findings presented emphasize the high safety limits of the vaccine used, but also highlight its poor immunogenicity.