We studied the coagulation profiles of 14 patients with advanced malignancies treated with Interleukin-2 (IL-2). A 43% prolongation of the PTT (P < 0.001) and a significant decrease in functional levels of factors II, IX, X, XI, and XII were observed 6 h post IL-2 treatment in comparison to pretreatment values. These parameters normalized within 2-3 d following IL-2 administration. The PT, factors V, VII, VIII, fibrinogen and D-dimer levels were unchanged with IL-2 treatment. This pattern of coagulopathy has not previously been reported.