BACKGROUND: Treatment of unruptured intracranial aneurysms (UIA) less than 7 mm is controversial. We created a scoring system to guide decision making and validated the system utilizing over 700 cases treated with microsurgery or endovascular procedures. METHODS: The scoring system SMALLSS included Size, (4-7 mm: 1 point, <
3.9 mm: 0 point), Multiple aneurysms (yes: 1, no: 0), Anatomic location (posterior: 1, anterior: 0), Lineage- family history of aneurysm (yes:1, no: 0), Lifetime risk (age <
65: 1, age >
65: 0), Smoking history (yes: 1, no: 0 ), Shape (irregular:1, smooth:0 ). Validation of this system was performed through retrospective review of prospectively maintained data for UIA patients treated between 2014 and 2021. We then performed an external validation of a cohort of 200 small aneurysms <
7 mm treated at a high volume cerebrovascular center. RESULTS: A total of 1152 cases with unruptured intracranial aneurysms were treated over the study interval, of which 771 aneurysms (66.9%) were under 7 mm, with the majority (64.3%) having SMALLSS score more than 2. Serious neurologic complications occurred in 18 out of 771 aneurysms (2.33%) of which 4 were hemorrhagic and 14 were ischemic. The obliteration rate was 98% in microsurgery group, 89.7% in endovascular group. During this same interval, 1126 patients with aneurysms <
7 mm were evaluated and not offered treatment, with 74.7% SMALLSS scores of 2 and under. External validation data showed no significant deviations from our cohort with 150 out of 200 aneurysms (75%) with a SMALLSS score of 2 or greater. CONCLUSION: The SMALLSS scoring system can be a good tool to guide treatment decision making for patients with small UIA. The risk of treatment did not increase with the projected increased risk of rupture and overall high obliteration rates were achieved with endovascular and surgical techniques.