The purpose of this study was to report patients with pharmacoresistant West syndrome of unknown cause whose magnetic resonance imaging (MRI) with diffusion weighted imaging (DWI) showed a transient decrease of diffusion in subcortical structures. Of 20 patients investigated over a 2-year period, three males and three females constitute the present series. They had daily clusters of infantile spasms with hypsarrhythmia for 4 to 24 months before the first investigation. Four were severely hypotonic. All aetiological studies involving intermediary metabolism, peroxysomes, mitochondria, and neurotransmitters in cerebrospinal fluid were negative. Patients underwent DWI when first examined at the mean age of 13 months, and on follow-up examination 6 to 18 months later. Diffusion was decreased in the pallidi and posterior brainstem. It was also decreased for five patients in thalami and for three in dentate nuclei. Repeat MRI, performed when spasms were still present but hypsarrhythmia had ceased, did not show the same abnormalities. Because of recruitment bias, this series probably overestimates the true incidence of such DWI abnormalities. The eventuality of toxic lesions, including some inborn error of metabolism or drug toxicity, is considered unlikely although it could not be excluded. The contribution of the epileptic encephalopathy itself appears the most likely course.West syndrome (WS), the most frequent epileptic encephalopathy, comprises epileptic spasms, psychomotor regression, and hypsarrhythmia. 1 In symptomatic cases, lesions may involve the cortex, the white matter, and the subcortical grey matter. Although conventional magnetic resonance imaging (MRI) discloses aetiology in 90% of cases, 2 a sizeable proportion remain unexplained, including those with poor outcome. In addition, the link between abnormalities disclosed on MRI and clinical symptomatology remains unclear in most instances.All abnormalities disclosed by neuroimaging are not necessarily related to the aetiology of WS. Indeed, steroid therapy produces apparent brain atrophy. 3 Diffusion weighted imaging (DWI) is sensitive to the molecular motion of water and enables cytotoxic oedema to be distinguished from vasogenic oedema. It is widely performed to evaluate ischaemic stroke and is increasingly used in other diseases such as neoplasms, inflammatory diseases, and traumatic injury. In the context of epilepsy, partial status epilepticus with reversible DWI images has been reported. 4 We now report a series of patients in which diffusion weighted magnetic resonance showed abnormalities in the subcortical grey matter in which it is not clear whether they result from the aetiology, from the epileptic encephalopathy itself, or from its treatment. MethodPatients with WS referred to our department without identified aetiology undergo axial and coronal, T 1 , T 2 , and fluid-attenuated inversion recovery (FLAIR) MRI, 2 to 4 months after onset of the disease, repeated after the age of 18 months in case of pharmacoresistance. In 2003, a first patient ...