Under continuous observation of several months, 42 patients from the eastern province of Sierra Leone, Liberia (Lofa County), and neighbouring Guinea were identified as Lassa fever cases by indirect immunofluorescent antibody technique, indicating that the disease is endemic in these areas. The clinical course varied from mild disease to severe illness with haemorrhagic disorders. The fatality rate was 14%. The occurrence of only two possible secondary cases suggests that person-to-person spread of the disease is unimportant epidemiologically. There was a wide range of patients' ages, tribes, and occupations, including a 2 months old baby and a white US citizen. Clinical, laboratory, and histopathological investigations demonstrated the panorganotropism of Lassa virus. Haematological tests in few selected haemorrhagic cases with Lassa fever did not support coagulation disorders or thrombocytopenia as causing the bleeding tendency. The histopathologic changes bear resemblance to those observed in Argentinian and Bolivian haemorrhagic fever, both being caused by viruses of the Arena group. However, Lassa virus hepatitis may be differentiated from liver lesions occurring in yellow fever, Marburg virus disease, and Ebola (Maridi) haemorrhagic fever.