Serum concentrations of interferon alfa, interleukin 1, and tumour necrosis factor alpha were measured in 25 untreated patients with systemic lupus erythematosus (SLE). A close correlation was found between serum concentrations of interferon alfa and the degree of fever, while no significant correlations were found between fever and interleukin 1 or tumour necrosis factor alpha. These results suggest the possible involvement of interferon alfa in the pathogenesis of fever in SLE.