Cerebral blood flow was studied with single photon emission computed tomography in seven girls with Rett syndrome at the John F. Kennedy Institute, Glostrup, Denmark and the Department of Clinical Physiology and Nuclear Medicine, Bispebjerg Hospital, Copenhagen, Denmark. Compared to results in an age matched control group of nine normal children, global cerebral blood flow was significantly lower in patients with Rett syndrome (54 vs 69 mL/100 g per minute). The blood flows in prefrontal and temporoparietal association regions of the telencephalon were markedly reduced, whereas the primary sensorimotor regions were relatively spared. The cerebral blood flow distribution in Rett syndrome was similar to the distribution of brain metabolic activity in infants of a few months of age. The most striking difference between the Rett syndrome and control groups was the pronounced frontal hypoperfusion in the Rett syndrome group; patients had a 30% lower anteroposterior flow ratio than the control group. These changes were not reflected in CT scans which showed cortical and central atrophy only in two of the seven patients. The age range of the patients was between 6.7 and 17.9 years with a median of 10.1 years. [1]