Data on the growth, regression, and de novo formation of arteriovenous malformations (AVMs) suggest that some of these lesions are not formed and developed only during embryogenesis. Patients with hereditary hemorrhagic telangiectasia (HHT) have a genetic propensity to form AVMs. The authors report on the growth and regression of AVMs in a single patient with HHT. This 26-day-old boy with a family history of HHT1 and a mutation in ENG on chromosome 9 presented with a generalized seizure. Results of computed tomography revealed a left frontoparietal intraparenchymal hemorrhage. Cerebral angiography revealed multiple AVMs. Follow-up angiograms obtained 5 months later showed both growth and regression of the AVMs. A craniotomy was performed for complete resection of the left parietal AVM. Histopathological features of the surgical specimen were examined. Active angiogenesis, as indicated by increased endothelial proliferation, might be a part of the underlying pathophysiology of the growth and regression of AVMs.