To classify the cerebral cavernous malformations and to investigate the natural history of cavernous malformations according to the classification, 41 patients with 61 cavernous malformations (40 caver-nous malformations from 22 patients treated with gamma knife surgery) were regularly followed up us-ing magnetic resonance (MR) imaging for a mean period of 25.5 months in treated cavernous malforma-tions and 20.7 months in untreated cavernous malformations, respectively. Cavernous malformations were classified into four types: type I, extralesional gross hemorrhage beyond cavernous malformation; type II, mixture of subacute and chronic hemorrhage; type III, area of hemosiderin with small central core; and type IV, area of hemosiderin deposition without central core. Follow-up MR images were ana-lyzed to evaluate changes in size, signal intensity, rebleeding, and perilesional adverse reaction of ir-radiation. A total of 61 cavernous malformations including 17 in type I, 23 in type II, 10 in type III, and 11 in type IV showed usual degradation of blood product in 22 cavernous malformations, no change in shape and signal intensity in 31 cavernous malformations, and eight cavernous malformations with rebleedings in the serial MR images. In these eight cavernous malformations with rebleedings, six oc-curred in type II and two in type III, but none in type I or IV. Rebleedings were more frequent in type II than in other types (p=0.044). Adverse reaction of irradiation was observed in five of 22 patients treat-ed with gamma knife surgery. Although most cerebral cavernous malformations showed evolution of hemorrhage or no change in size or shape on follow-up MR images, cerebral cavernous malformations represented as mixture of subacute and chronic hemorrhage with hemosiderin rim (type II) have a higher frequency to rebleed than other types of cerebral cavernous malformations. Cerebral cavernous malformations represented as hemosiderin deposition without central core (type IV) have a lower ten-dency to rebleed than other types and do not need any treatment. Most of the adverse reaction of irradia-tion after gamma knife surgery around cavernous malformations are transient findings and are consi-dered to be perilesional edema.