Purpose: We evaluated the safety of bolus infusion through intraosseous access using the EZ-IO in adults in terms of extraosseous flow. Methods: We conducted a prospective study of adults (over age 18) in whom intraosseous access through the tibia was performed by emergency physicians or residents from June 2010 to June 2011. We used ultrasonography to confirm extraosseous flow during infusion of 80 mL normal saline with a 4 mL/s flow rate through intraosseous access, immediately after confirmation of intraosseous needle insertion using conventional methods. Finally, we recorded any complications that occurred immediately in the area of intraosseous access. Results: Of 30 patients enrolled in the study, 22(73.3%) were male and eight (26.7%) were female. The mean age of study participants was 62.78±15.68 years; mean cortical thickness of participants` tibias was 0.27±0.03 cm. The mean time required for performance of the intraosseous access procedure was 16.00±4.65 s; success rate on the first attempt, 100.0%. No immediate complications, including swelling or extraosseous flow at the area of intraosseous access, were observed. Conclusion: In this study, bolus infusion though intraosseous access using the EZ-IO in adults in emergency departments was a safe option in terms of extraosseous flow.