We report five paediatric cases of portal vein thrombosis (PVT) occurring during chemotherapy, observed in two institutions over an 8-year time period. These children aged 2.5-15 years were treated for Burkitt's lymphoma, Ewing's tumour, small cell bone tumour or medulloblastoma. PVT was diagnosed on colour Doppler ultrasonography (US). In four patients, thrombosis occurred 2-45 days after severe hepatic veno-occlusive disease (HVOD) secondary to intensive chemotherapy containing busulfan. In one case, PVT occurred in the absence of HVOD in a patient with pre-existing periportal lymphomatous infiltration. Four patients experienced persistent portal hypertension, which resulted in death in one. PVT during chemotherapy in children is a rare event and appears to be closely related to intensive chemotherapy containing busulfan and to be associated with HVOD.