Follow-up studies of forty patients with forty-one medullary venous malformations (MVM's) are described. The patients included nineteen males and twenty-one females in an age range from eight to seventy-two years (mean 43.4 years). All cases were diagnosed angiographically. Of the forty-one lesions, twenty-five were located in supratentorial and sixteen in infratentorial regions. In twenty-five supratentorial MVM's, thirteen had superficial drainage, eight deep drainage and three both superficial and deep drainage. Fifteen patients presented with intracranial hemorrhage and eleven patients with complaints or symptoms not related to hemorrhage. Fourteen hemorrhages were found incidentally. The mean follow-up period was three years and eleven months and the longest was sixteen years. No patient died of recurrent hemorrhage and only two had recurrent hemorrhage after a short interval. One patient died of malignant glioma. Thirty-seven out of thirty-nine patients (95%) were in an excellent or good clinical condition with a score above 80% on the Karnofsky Performance Test. This study indicates the low incidence of rebleeding in ruptured MVM's (2/15, 13.3%) and of bleeding in incidentally found unruptured MVM's (0/25, 0%). However, this evidence may be sufficient to support aggressive radical surgery for MVM's.