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 cavernous malformations from 22 patients 
treated with gamma knife surgery) were regularly followed up using magnetic 
resonance (MR) imaging for a mean period of 25.5 months in treated cavernous 
malformations 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 analyzed to evaluate changes in size, signal intensity, 
rebleeding, and perilesional adverse reaction of irradiation. 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 occurred 
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 treated 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 tendency to rebleed than other types and do not need any 
treatment. Most of the adverse reaction of irradiation after gamma knife surgery 
around cavernous malformations are transient findings and are considered to be 
perilesional edema.