Introduction: On Denmark's decision to join the war in Iraq, it was decided to immunise all personnel to be deployed there with anthrax vaccine. This paper describes the types and frequency of adverse reactions to the vaccine., Materials and Methods: Between March 2003 and February 2004, 1,899 immunisations against anthrax in 755 personnel were registered at the health care facilities of the Danish Defence., Results: No serious or life-threatening adverse reactions were reported. The data showed a statistically significant decrease in adverse reactions between the four immunisation series. For males, adverse reactions after the second immunisation depended on possible adverse reactions to the first immunisation. In females, this correlation was not found. No differences in the frequency of adverse reactions between males and females were found., Discussion: None of the vaccinated persons suffered serious or life-threatening adverse reactions. Compared to other studies on self-reported adverse reactions, the overall frequency was low. The data showed a statistically significant decrease in adverse reactions between the four immunisation series. This has been described in comparable studies. The correlation between adverse reactions in the second immunisation being dependent on adverse reactions to the first immunisation has been partly confirmed by other studies. No differences between the sexes were found, probably because only a few females were included in the study. In case of a significant level of threat concerning biological weapons, immunisation against anthrax is considered appropriate.