The incidence of thrombotic complications chronologically related to L-asparaginase administration is retrospectively analyzed in 238 adult ALL patients treated according to the GIMEMA protocol ALL 0288. The patients (126 males and 112 females, aged 12-68 years, median 29) received E. coli L-asparaginase (L-ase) in the induction phase at a dosage of 6000 U/m2/day x 7 d starting on d 15, as well as vincristine, prednisone, daunorubicin and cyclophosphamide, the last-named by random 1:1. Ten patients (4.2%) developed thrombotic complications 5-15 d (median 11 d) after the start of L-ase treatment. The thrombotic events, which were lethal in 5 patients, involved the cerebral sinus (5 cases), the cerebral arteria (2 cases), the portal vein (1 case), the pulmonary district (1 case), and a deep vein in the lower extremity (1 case). The occurrence of these complications was not related to the general thrombotic risk factors, nor to the main clinical and laboratory data registered at diagnosis and immediately before the start of L-asparaginase treatment. The present study documents for the first time in a sufficiently large series of adult ALL patients that the incidence and the severity of thrombotic events related to L-ase administration are relevant and need further consideration.