AIM: To evaluate the relationship between optic nerve sheath diameter (ONSD) and intracranial pressure (ICP) in patients with acute ICP elevation controlled through external ventricular drainage (EVD). MATERIAL and METHODS: In this prospective observational study, simultaneous measurements of right and left ONSD, ICP, and cerebral perfusion pressure (CPP) were recorded in 46 adult patients undergoing EVD. Two patients were excluded due to ophthalmic pathology and missing data, resulting in a final cohort of N = 44 patients. After we obtained the baseline measurements, 5 mL of cerebrospinal fluid (CSF) were drained, and the measurements were repeated 2 minutes (min) later. RESULTS: CSF drainage led to significant reductions in both ICP and bilateral ONSD (p = .001 for all), while CPP significantly increased (p = .038). We identified an ONSD cut-off of 5.35 mm for detecting ICP values ≥15 mmHg, and this cut-off had 53% sensitivity and 86% specificity (Youden index: 0.395). However, we could not determine a reliable cut-off for detecting ICP values ≥20 mmHg (p = .076). CONCLUSION: When ICP is controlled through EVD, the linear relationship between ONSD and ICP observed during acute pressure elevations may be disrupted, particularly at higher ICP values (≥20 mmHg). Although we identified a cut-off ONSD value of 5.35 mm for detecting ICP values ≥15 mmHg, the low sensitivity of this cut-off significantly limits its clinical utility. Additionally, since no reliable cut-off could be established for ICP values ≥20 mmHg, ONSD may have limited value for monitoring pressure following CSF drainage.