To determine whether travellers from malaria-free countries can develop unrecognised or asymptomatic malaria after visiting endemic areas, we analysed data from 751 prospective blood donors who had visited or resided in malaria-risk areas in the previous 3 years. Malarial antibodies were measured using an established commercial enzyme-linked immunoassay incorporating four recombinant blood-stage plasmodial antigens and with published sensitivity >= 83% and specificity 100%. Details of countries visited and antimalarial chemoprophylaxis used were obtained by questionnaire. Among the 606 subjects resident in malaria-free countries and with no past history of malaria, 176 had visited high-risk countries as categorised by the World Health Organisation (WHO). Of these, 89 took no chemoprophylaxis including 6 (6.7%) who were antibody-positive; there were no antibody-positive subjects in the 87 who took chemoprophylaxis (P = 0.029), which was that recommended by WHO in 84% of cases. These data underscore the value of effective antimalarial prophylaxis in non-immune travellers visiting high malaria-risk areas but also suggest that unrecognised infections can occur in those unprotected by chemoprophylaxis. (c) 2006 Published by Elsevier B.V.