Allogeneic bone marrow transplantation is frequently associated with neurological complications, particularly intracerebral bleeds and infections. Cerebral venous sinus thrombosis has only rarely been reported following allogeneic transplants. We report three cases of cortical venous thrombosis following allografting for acute lymphoblastic leukaemia. Two patients received marrow from HLA-identical siblings and one from an unrelated donor. Two of the patients presented with grand mal seizures and one presented with a headache. No neurological abnormalities were found upon clinical examination and lumbar puncture was normal in all three cases. In two of the patients computed tomography (CT) of the brain was normal and in the third showed non-specific abnormalities. Magnetic resonance imaging (MRI) with MR angiography (MRA) demonstrated cerebral venous sinus thrombosis in all three patients. In conclusion, cerebral venous sinus thrombosis should be considered in the differential diagnosis when neurological symptoms occur following allogeneic bone marrow transplantation. We therefore advocate the use of MRA for unexplained neurological symptoms post-allograft since without it cerebral venous sinus thrombosis may easily be missed.