OBJECTIVE: We investigated whether air in the cisterns or ventricles on postoperative computed tomography (CT) (reflecting the opening of the cerebrospinal fluid spaces during surgery) is a predictor of classical or nodular leptomeningeal disease (LMD) after resection of brain metastases. METHODS: We retrospectively analyzed 73 patients who underwent gross total resection of brain metastases between 2012 and 2020. Patients with air in the cisterns or ventricles on postoperative day 1 CT were categorized into the air-positive group, whereas those without air in the cisterns or ventricles on postoperative day 1 CT were categorized into the air-negative group. The primary outcome was the occurrence of classical or nodular LMD (nLMD), which was assessed using survival analysis. RESULTS: There were 15 (21%) patients in the air-positive group and 58 (79%) in the air-negative group. The air-positive group exhibited significantly more cerebellar and ventricular contact lesions than the air-negative group. The 4-year rate of classical or nLMD was significantly higher in the air-positive group than in the air-negative group (67% vs. 33%, P <
0.001). Multivariate analysis identified air in the cisterns or ventricles on postoperative CT as the only significant predictor of classical or nLMD (P <
0.001). CONCLUSIONS: Postoperative air in the cisterns or ventricles can predict early classical or nodular leptomeningeal disease.