One hundred and fifty-three British soldiers and 86 Royal Air Force (RAF) personnel were deployed on a hostage rescue operation in Sierra Leone. For 3 days they were exposed to various infection risks and 6 weeks later some of the soldiers presented with gastrointestinal complaints. Both groups were screened with structured questionnaires, blood investigations and (where indicated) faecal microscopy and charcoal culture for helminths. Definite and probable cases of helminth infection were treated with albendazole and all soldiers were screened again after 3 months. Among the soldiers investigated, 73/145 (50%) reported gastrointestinal symptoms and 70/139 (50%) had eosinophilia. Among these, 17/66 (26%) had hookworm infection, 6/66 (9%) had Strongyloides stercoralis infection and 1/66 (2%) had Giardia lamblia infection. Eosinophilia was most strongly associated with entering the enemy camp and being in the platoon that attacked the area around the camp latrines. Among RAF personnel, who were not involved in activities on the ground, 3/86 (3%) had borderline eosinophilia. Treatment of 105/153 (69%) soldiers with albendazole was well tolerated and, on follow-up screening 3 months later, 23/124 soldiers (19%) had gastrointestinal symptoms and 18/121 (15%) had eosinophilia. Faecal investigations and schistosomiasis serology tests were all negative at this stage.