L-Asparaginase treatment during induction therapy in acute lymphoblastic leukaemia (ALL) is known to be frequently complicated by thromboembolic events. It was recently suggested that L-asparaginase derived from Erwinia chrysanthemi alters the coagulation system less severely than does Escherichia coli asparaginase. In a series of 11 adult patients with ALL, we investigated some parameters of the coagulation system during treatment with Erwinia asparaginase. The doses employed were rather high; all patients below the age of 60 years received 15,000 U/m2 daily over 14 days. In accordance with what is known from treatment with E. coli asparaginase, we observed significant lowering of antithrombin as well as of fibrinogen. However, as to fibrinogen indeed a significant decrease had occurred prior to the institution of Erwinia asparaginase treatment. The most striking observation in the present study was that the levels of prothrombin complex, reflecting the function of K-vitamin dependent coagulation factors II, VII and X, remained within normal ranges during treatment. This indicates that these coagulation factors were not affected by Erwinia asparaginase, an observation at variance with several reports where E. coli asparaginase was investigated. This latter observation was the only finding which could lend support to the view that Erwinia asparaginase affects the coagulation system less than E. coli asparaginase. Finally, one of our patients developed a sinus thrombosis, a severe thrombotic complication.