The use of cyclosporin A has been associated with several side-effects, including neurotoxicity. The mechanism of toxicity is not well known. We report two patients treated with cyclosporin A who developed lesions in the cerebral white matter associated with abnormally elevated cerebral blood flow velocities on transcranial doppler ultrasound and abnormal vascular appearance on magnetic resonance angiography. Bone Marrow Transplantation (2000) 26, 801-804.