To assess the relationship between serum cytokine behavior and treatment outcome in type 1 autoimmune hepatitis, serum levels of interferon-gamma, interleukin-2, interleukin-4, and interleukin-10 were measured by enzyme immunoassay in 43 patients and 20 normal subjects. Serum samples were similarly tested in 38 patients after corticosteroid treatment. Serum levels of interleukin-2 and interleukin-4 were significantly lower in patients than in normal subjects. Interleukin-2 was the least common cytokine detected before (3%), during (0%), or after treatment (0%). Serum levels of interleukin-10 at presentation did not differ from those of normal subjects but they did decrease during therapy, especially in patients who entered remission. Changes in these levels, however, did not always parallel treatment outcome or histological activity. We conclude that serum levels of interleukin-2 and interleukin-4 are lower than normal in type 1 autoimmune hepatitis. Serum concentrations of interleukin-10 diminish during corticosteroid therapy but changes do not closely reflect outcome. The rarity of interleukin-2 in serum may be a distinguishing feature.