OBJECTIVE: The authors present cases of cyst formation after gamma knife radiosurgery for arteriovenous malformation. METHODS: One hundred seventy eight arteriovenous malformation patients were treated with Gamma knife radiosurgery from October, 1994 to October, 2002. Six patients developed cyst after Gamma knife radiosurgery after a mean of 46.5 months(range, 11-62 months). Male to female ratio was 5: 1 and mean age was 19.7 years(range, 15-24 years). RESULTS: The mean marginal dose was 22.7Gy(range, 10-25Gy) and mean volume was 22.4cm3(range, 1.9-80.3cm3). Initial symptoms were hemorrhage in three, seizure in two and headache in one. Symptom after cyst formation was motor weakness in three patients, asymptome in three patients. The locations of the cyst were basal ganglia in two, parietal in two, occipital in one, parietooccipital in one. Ventriculoperitoneal shunt was performed in one, Ommaya's reservoir insertion in one, craniotomy and removal of cyst and then Ommaya's reservoir insertion after 8 months in one and no treatment was given in three. CONCLUSION: Cyst formation after Gamma knife radiosurgery for arteriovenous malformation may occur. However, symptomatic cysts can be effectively treated with Ommaya's reservoir insertion, or cystoperitoneal shunt and in some cases, excision is needed.