Three cases of deep cerebral vein thrombosis presenting as encephalitic illnesses are described. Thyrotoxicosis was present in one case, ulcerative colitis in one case and an anticardiolipin antibody was detected in two cases. All three patients were on oestrogen and progesterone. Magnetic resonance imaging and angiography allowed rapid confirmation of the diagnosis and permitted non-invasive follow up of this condition. The first two patients made complete clinical recoveries despite having thalamic infarction, in one case bilaterally, demonstrable radiologically.