OBJECTIVE: To propose a modified classification system for spontaneous intracranial dissecting aneurysms (IDAs) that can guide treatment decisions. METHODS: Between January 2010 and December 2015, 3183 consecutive patients were referred to our department for endovascular treatment of a cerebral aneurysm. Among this group, 309 patients harboring a total of 323 spontaneous IDAs were identified. Based on a modified imaging classification system, spontaneous IDAs were classified into 4 subtypes: classic dissecting aneurysm, segmental ectasia, dolichoectatic dissecting aneurysm, and large mural bleeding ectasia. A logistic regression analysis was conducted to identify predictors of clinical outcomes. RESULTS: Of the 323 IDAs, 216 (66.87%) were treated with stent-assisted coiling, 47 (14.55%) with internal trapping, and 60 (18.58%) with sole stenting. Of the 309 patients, 8 (2.59%) suffered intraoperative complications, 40 (12.94%) experienced postoperative complications, 9 died (2.79%), and 1 (0.32%) had severe disability. The mean duration of clinical follow-up of the 309 patients was 10.40 months (range, 3-72 months). Imaging follow-up was available for 262 patients (274 IDAs), with a mean duration of 7.10 months (range, 3-60 months), and 24 aneurysm recurrences (9.16%) were noted. Aneurysm type was the sole independent predictor of different outcomes on logistic regression analysis (P = 0.004). CONCLUSIONS: There was a strong relationship between the subtypes of spontaneous IDAs and clinical course. Our classification system is confirmed to be helpful in assessing patients' prognosis and guiding their treatment.