Prophylaxis of malaria and frequency of clinical attacks were studied in a group of Nigerian children who were born abroad and recently returned home. Two control groups of young children, born and resident in Nigeria, were studied in the same way: one group was protected against malaria and the other was not. Malarial immunofluorescent antibody (IFA) was measured in all 3 groups. All the groups had high IFA positivity rates of 79% or over, but titres were low in the children born abroad, higher in the Nigerian-born protected children, and highest in the unprotected group. Frequency of clinical malaria was the same in both groups of protected children whether born abroad or locally. There appeared to be no long-term advantage in giving regular malaria chemoprophylaxis to children of semi-immune mothers returning permanently to malarious countries, if the parents were alert to provide prompt treatment of clinical malaria.