Crimean-Congo haemorrhagic fever (CCHF) was diagnosed in 8 patients; 7 were staff members at Tygerberg Hospital who had been infected by a patient in whom the disease had not initially been diagnosed. Two patients, the initial case and a staff member, died and 4 became seriously ill. The immunopathogenesis of CCHF appears to be multifactorial. Certain features were common to all patients--leucopenia, thrombocytopenia, elevated liver enzyme values and low serum total protein levels. Ultrastructural changes in and around skin capillaries, including intracytoplasmic endothelial tuboreticulated bodies, were found. Virus-like particles were found on electron microscopy. Important individual factors related to prognosis were identified. The patients who survived all mounted a good antibody response, and manifested no coagulation defect extensive enough to explain the haemorrhagic tendency. In the patients who died no evidence of antibody production was detected; both developed diffuse intravascular coagulation and in 1 evidence of immune complex formation and complement consumption was found. Hepatorenal failure and cardiovascular collapse characterized the terminal period. Early clinical recognition of CCHF with specific attention to factors amenable to treatment may vastly improve the prognosis.