Laboratory studies were performed on 128 children clinically diagnosed as measles when seen at the Infectious Diseases Hospital, Kenyatta National Hospital (IDH), Nairobi (86 cases) and the Rural Health Training Centre, Maragua, Central Province (42 cases) between 9 July and 31 August 1984. A concurrent measles infection was confirmed in 95% of the children seen at IDH and in 85% of those seen at Maragua, with similar proportions of confirmations in children who had, and who had not, received measles vaccine. No differences in the number of sero-conversions nor in the absolute levels of acute or convalescent HI antibody titres could be detected between vaccinated and unvaccinated children. Analysis of the cases seen at Maragua indicates that about two thirds of the children who had received vaccine were protected. A pilot study of vaccinating children at 8 months and again at 12–13 months is suggested in an attempt to eradicate measles.