Optic nerve sheath diameter (ONSD) seen on ocular US has been associated with increased intracranial pressure (IICP). However, most studies have analyzed normal range of ONSD and its optimal cut-off point for IICP in Caucasian populations. Considering ONSD differences according to ethnicity, previous results may not accurately reflect the association between IICP and ONSD in Koreans. Therefore, we conducted this study to investigate normal range of ONSD and its optimal threshold for detecting IICP in Korean patients.This prospective multicenter study was performed for patients with suspected IICP. ONSD was measured 3 mm behind the globe using a 13-MHz US probe. IICP was defined as significant brain edema, midline shift, compression of ventricle or basal cistern, effacement of sulci, insufficient gray/white differentiation, and transfalcine herniation by radiologic tests. The results of the ONSD are described as the median (25th-75th percentile). The differences of ONSD according to disease entity were analyzed. A receiver operator characteristic (ROC) curve was generated to determine the optimal cut-off point for identifying IICP.A total of 134 patients were enrolled. The patients were divided into 3 groups as follows: patients with IICP, n = 81 (60.5%); patients without IICP, n = 27 (20.1%); and control group, n = 26 (19.4%). ONSD in patients with IICP (5.9 mm [5.8-6.2]) is significantly higher than those without IICP (5.2 mm [4.8-5.4]) (P < 0.01) and normal control group (4.9 mm [4.6-5.2]) (P < 0.001). Between patients without IICP and normal control group, the difference of ONSD did not reach statistical significance (P = 0.31). ONSD >5.5 mm yielded a sensitivity of 98.77% (95% CI: 93.3%-100%) and a specificity of 85.19% (95% CI: 66.3%-95.8%).In conclusion, the optimal cut-off point of ONSD for identifying IICP was 5.5 mm. ONSD seen on ocular US can be a feasible method for detection and serial monitoring of ICP in Korean adult patients.