Hematoma expansion in small/medium-sized acute epidural hematoma (AEDH) cases upon emergency admission is critical. Predicting hematoma expansion can lead to early surgical interventions, improving outcomes and eliminating the need to check for expansion via computed tomography (CT). This study aimed to identify the most reliable predictors of AEDH expansion. We retrospectively collected data from patients with pure AEDH not requiring surgical treatment upon emergency admission from 2012-2022. We assessed clinical and laboratory data, time from injury to the first CT, and time to follow-up CT. Factors predictive of hematoma expansion on the second follow-up CT, including the leakage sign (LS), were analyzed. A total of 23 patients with pure AEDH without surgery at admission were included, and LS was positive in 18. Thirteen patients showed hematoma expansion. The hematoma expansion group showed a significantly higher rate of positive LS and lower mean platelet count than the group without hematoma expansion. LS's predictive value for AEDH expansion showed 100% sensitivity and 50% specificity. All patients with negative LS and normal platelet counts showed no hematoma expansion. Analyzing the time from injury to the first CT suggested that LS (+) within 120 min strongly predicted hematoma expansion. Reconstructed three-dimensional images of the leakage point on the skull revealed multiple mottled bleeding points on the dural surface. LS can predict hematoma expansion in patients with pure AEDH for whom emergency surgery is unnecessary at admission. The time from injury and platelet counts must also be considered.