Background and Purpose— Our aim was to describe variables associated with initial misdiagnosis of subarachnoid hemorrhage (SAH). We also analyzed the relationship of misdiagnosis with poor outcome and complications in good Hunt and Hess (HH) cases. Methods— In a prospective cohort of 401 patients with SAH, misdiagnosis was defined as failure to correctly identify, at first physician contact, a subsequently documented SAH; this meant no urgent radiological study and lumbar puncture was performed. Poor outcome was defined as modified Rankin Scale score 3 to 6 at 3-month follow-up. We recorded age, sex, hypertension, diabetes mellitus, current smoking, previous antithrombotic treatment, initial HH and radiological severity, presence of aneurysm, first therapeutic procedure, hydrocephalus, delayed cerebral ischemia (DCI), rebleeding, and procedure-related complications. Results— Misdiagnosis was confirmed in 104/401 (25.9%) patients, who also had a longer time-to-admission to hospital. Misdiagnosis was associated with less clinical and radiological severity, compared with a correct diagnosis; the 2 groups did not differ in age or cardiovascular risk factor profile. Poor outcome was registered in 167/401 patients (41.6%). Age, misdiagnosis, and greater clinical and radiological initial severity were independent predictors of poor outcome. In the 236 patients (58.8% of cohort) with HH 1–2, misdiagnosis was associated with poor outcome in univariate and multivariate analysis, respectively (odds ratio=3.89; 95% CI, 1.89–8.01). Delayed cerebral ischemia (odds ratio=2.47; 95% CI, 1.2–5.09) and procedure-related complications (odds ratio=2.27; 95% CI, 1.07–4.82) were independently associated with misdiagnosis. Conclusions— Misdiagnosis is an unresolved problem in SAH, and it is a missed opportunity for good outcome in patients with HH 1–2. The poor outcome is partially explained by a higher risk of delayed cerebral ischemia and procedure-related complications in misdiagnosed patients. There is a need to improve the diagnostic strategy in patients reporting only a headache (HH 1–2) after SAH.