Malaria is usually the most common cause of fever in children under 5 years of age in these areas. Antimalarial treatment should therefore be given to children with fever (>37.5 °C) or a history of fever and no other obvious cause. Malaria is the most likely cause of their illness and there is as yet no evidence to show that the benefits of parasitological diagnosis in this highly vulnerable group outweigh the risks of not treating false negatives. In children of 5 years of age and above, malaria becomes progressively less likely as a cause of fever, as immunity is acquired. In these older children and in adults, malaria diagnosis should be based on a parasitological confirmation. Parasitological diagnosis should be promoted in pregnant women, to improve the differential diagnosis of fever and to reduce unnecessary use of antimalarials in pregnancy. Parasitological diagnosis is also particularly important in settings with a high prevalence of HIV/AIDS because of the high incidence of febrile disease that is not malaria in HIV-infected patients.