BACKGROUND: A ruptured vertebral artery dissecting aneurysm (VADA) with a high clinical grade (Hunt and Hess grade 5) has a devastating prognosis. Because of the high rebleeding rate and location, rapid mortality can occur in patients owing to brainstem compression. Adjuvant decompression of the posterior fossa after securing the aneurysm may improve the outcomes of these patients. METHODS: Between January 2011 and December 2016, 22 patients who presented with Hunt and Hess grade 5 ruptured VADA underwent endovascular treatment. Patients were divided into 2 groups: group 1 (n = 12) received conventional endovascular treatment of VADA and external ventricular drainage, and group 2 (n = 10) received external ventricular drainage and suboccipital craniectomy for posterior fossa decompression after rapid endovascular trapping of VADA. RESULTS: In group 2, the survival rate and good clinical outcome rate (modified Rankin scale score <= 2) were 80% and 60%, respectively, which were favorable to the corresponding rates in group 1 (66.67% and 16.67%). Moreover, 80% of patients (8/10) in group 2 regained consciousness compared with 50% of patients in group 1. CONCLUSIONS: Adjuvant decompression of the posterior fossa can improve outcomes in patients with Hunt and Hess grade V ruptured VADA.