Background and purpose: To evaluate the reliability and accuracy of nonaneurysmal perimesencephalic subarachnoid hemorrhage (NAPSAH) on Noncontrast Head CT (NCCT) between numerous raters. Materials and methods: 45 NCCT of adult patients with SAH who also had a catheter angiography (CA) were independently evaluated by 48 diverse raters; 45 raters performed a second assessment one month later. For each case, raters were asked: 1) whether they judged the bleeding pattern to be perimesencephalic; 2) whether there was blood anterior to brainstem; 3) complete filling of the anterior interhemispheric fissure (AIF); 4) extension to the lateral part of the sylvian fissure (LSF); 5) frank intraventricular hemorrhage; 6) whether in the hypothetical presence of a negative CT angiogram they would still recommend CA. An automatic NAPSAH diagnosis was also generated by combining responses to questions 2 -5. Reliability was estimated using Gwet 's AC1 ( K G ), and the relationship between the NCCT diagnosis of NAPSAH and the recommendation to perform CA using Cramer 's V test. Multi -rater accuracy of NCCT in predicting negative CA was explored. Results: Inter -rater reliability for the presence of NAPSAH was moderate ( K G = 0.58; 95%CI: 0.47, 0.69), but improved to substantial when automatically generated ( K G = 0.70; 95%CI: 0.59, 0.81). The most reliable criteria were the absence of AIF filling ( K G = 0.79) and extension to LSF ( K G = 0.79). Mean intra-rater reliability was substantial ( K G = 0.65). NAPSAH weakly correlated with CA decision ( V = 0.50). Mean sensitivity and specificity were 58% (95%CI: 44%, 71%) and 83 % (95%CI: 72 %, 94%), respectively. Conclusion: NAPSAH remains a diagnosis of exclusion. The NCCT diagnosis was moderately reliable and its impact on clinical decisions modest. (c) 2024 The Authors. Published by Elsevier Masson SAS. This is an open access article under the CC BY -NC -ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)