The clinical spectrum of Lassa fever is described and discussed in terms of the possible pathophysiological events involved. Early diagnosis is essential to permit prompt isolation of the potentially infectious patient. Lassa fever may be suspected on clinical grounds, but specific early diagnosis depends upon isolation of the virus. Virus isolation is best accomplished from serum obtained during the first 2 weeks of illness. The patterns of viraemia and virus excretion described in this paper are useful guidelines for determining the duration of patient isolation. Problems encountered in the isolation, management, and transport of the patient with Lassa fever are discussed.