Prostaglandin E1 was used to prevent veno-occlusive disease of the liver after allogeneic bone marrow transplantation for leukemia. It was given in continuous IV infusion from day -8 to day 30 after BMT at the dose of 0.3 microgram/kg/h. The patients were studied according to the risk factors of VOD: diagnosis, intensification of the conditioning and previous liver abnormalities. The diagnosis of VOD was made on at least two of the following factors: weight gain, hepatomegaly, jaundice, ascitis, pain of the right upper quadrant, increased platelet consumption. One hundred and nine patients were studied, 50 were treated by PGE1 and 59 did not receive it. Univariate analysis shows that the actuarial incidence of VOD was 12.2% in the PGE1 group and 25.5% in the non PGE1 group (P = 0.05). In acute leukemia, it was 39.1% in the non treated group and 12.8% in the PGE1 treated group (P = 0.02). Patients with previous hepatitis had an incidence of 62.5% in the non treated group and 15.5% in the treated group (P = 0.05). A positive CMV serology seemed to increase the risk of VOD: the incidence of VOD was 31.4% in non treated patients and 22% in PGE1 treated patients. The multivariate analysis of the risk factors of VOD show that unfavorable factors were: recipient positive CMV serology (P = 0.06), hepatic disease prior to transplant (P = 0.02) and the absence of PGE1 treatment (P = 0.02). This study suggests that prophylactic PGE1 treatment may decrease the incidence of VOD in patients at risk treated for leukemia by allogeneic bone marrow transplantation.