Background and Purpose— The purpose was to obtain a reliable treatment score for unruptured intracranial aneurysms (UIAs) from variables known at baseline. Methods— The series included 142 patients with UIAs diagnosed between 1956 and 1978 when UIAs were not treated and were followed up until the first aneurysm rupture, death, or the last contact. Previously published UIA treatment score was recorded, and finally, a new treatment score was constructed. Results— The median follow-up time was 21.0 years (interquartile range, 10.4–31.8 years). A total of 34 patients had an aneurysm rupture during 3064 person-years of follow-up. The UIA treatment score differed slightly between those with and without an aneurysm rupture (9.4±2.8 versus 8.3±3.1, P =0.082). The receiver operating characteristics curve of the UIA treatment score for predicting rupture showed a modest area under the curve (AUC; 0.618, 95% CI, 0.502–0.733; P =0.059). The best new treatment score consisted of 4 variables: age <40 years (2 points), current smoking (2 points), UIA size ≥7 mm (3 points), and location (anterior communicating artery, 5 points; internal carotid bifurcation, 4 points; and posterior communicating artery, 2 points). Scores of 5 to 12 points were associated with high cumulative UIA rupture rates (16%–60% at 10 years and 49%–80% at 30 years), favoring UIA treatment. Scores of 1 to 4 points (3% at 10 years and 18% at 30 years) favored conservative treatment and needed additional indications for treatment. Patients with a score of 0 points should not be treated (no ruptures during 513 follow-up years). The area under the curve for this scoring was 0.755 (95% CI, 0.657–0.853; P <0.001) and was better than that of the UIA treatment score ( P =0.02). Conclusions— This new simple and rapid scoring system is reliable for evaluating treatment indications with regard to the lifelong prevention of aneurysm rupture.