IMPORTANCE Rapid access to specialized care is recommended to improve outcomes after aneurysmal subarachnoid hemorrhage (SAH), but understanding of the optimal onset-to-treatment time for aneurysmal SAH is limited. OBJECTIVE To assess the optimal onset-to-treatment time for aneurysmal SAH that maximized patient outcomes after surgery. DESIGN, SETTING, AND PARTICIPANTSThis cohort study assessed 575 retrospectively identified cases of first-ever aneurysmal SAH occurring within the referral networks of 2 major tertiary Australian hospitals from January 1, 2010, to December 31, 2016. Individual factors, prehospital factors, and hospital factors were extracted from the digital medical records of eligible cases. Data analysis was performed from March 1, 2020, to August 31, 2021. EXPOSURES Main exposure was onset-to-treatment time (time between symptom onset and aneurysm surgical treatment in hours) derived from medical records. MAIN OUTCOMES AND MEASURES Clinical characteristics, complications, and discharge destination were extracted from medical records and 12-month survival obtained from data linkage.The associations of onset-to-treatment time (in hours) with (1) discharge destination of survivors (home vs rehabilitation), (2) 12-month survival, and (3) neurologic complications (rebleed, delayed cerebral ischemia, meningitis, seizure, hydrocephalus, and delayed cerebral injury) were investigated using natural cubic splines in multivariable Cox proportional hazards and logistic regression models. RESULTSOf the 575 patients with aneurysmal SAH, 482 patients (mean [SD] age, 55.0 [14.5] years; 337 [69.9%] female) who received endovascular coiling or neurosurgical clipping were studied. A nonlinear association of treatment delay was found with the odds of being discharged home vs rehabilitation (effective df = 3.83 in the generalized additive model, χ 2 test P = .002 for the 4-df cubic spline), with a similar nonlinear association remaining significant after adjustment for sex, treatment modality, severity, Charlson Comorbidity Index, history of hypertension, and hospital transfer (likelihood ratio test: df = 3, deviance = 9.57, χ 2 test P = .02). Both unadjusted and adjusted cox regression models showed a nonlinear association between time to treatment and 12-month mortality with the lowest hazard of death with receipt of treatment at 12.5 hours after symptom onset, although the nonlinear term became nonsignificant upon adjustment. The odds of being discharged home were higher with treatment before 20 hours after onset, with the probability of being discharged home compared with rehabilitation or other hospital increased by approximately 10% when treatment was received within the first 12.5 hours after symptom onset and increased by (continued) Key Points Question What is the optimal time between symptom onset and treatment after aneurysmal subarachnoid hemorrhage (SAH) to maximize patient outcomes? Findings In this cohort study, more favorable patient outcomes (discharge home and survival at 12 months) were observed wh...