The prognosis of Sturge-Weber syndrome (SWS) is partly related to early occurrence of seizures but the diagnosis of this phakomatosis may be difficult during the 1st year of life. We have performed a retrospective study of seven patients with confirmed SWS (age 7 days to 3 months). None of the patients was asymptomatic at the time of the study. They all underwent MRI (T1 and T2 sequences) and single photon emission computed tomography (SPECT) at the same time. Regional cerebral blood flow was measured using xenon-133. In all cases, myelination appeared to be accelerated in the areas underlying the leptomeningeal angioma on both MRI sequences. In five cases, SPECT showed hyperperfusion in the damaged hemisphere. In one case, the SPECT was symmetrical and in another it showed hypoperfusion in the damaged hemisphere which was already atrophied. These data suggest that the accelerated myelination is not related to ischemia but to transient hyperperfusion. This MRI pattern can be helpful for the early diagnosis of SWS, which is of utmost importance for preventive antiepileptic treatment.