BACKGROUND: Pulmonary aspiration of gastric contents during tracheal intubation is a life-threatening complication in emergency patients. Rapid sequence intubation is commonly performed to prevent aspiration but is not associated with low risk of intubation related complications. Although it has been considered that aspiration can be prevented in the lateral position, few studies have evaluated the ability to prevent aspiration. Moreover, this position is not always a favorable position for tracheal intubation. If aspiration can be prevented in a clinically relevant semi-lateral position, it may be advantageous. We assessed the ability to prevent aspiration in the lateral position and various degrees of the semi-lateral position using a vomiting–regurgitation manikin model. METHODS: A manikin's head was placed in the neutral, simple extension, or sniffing position. The amount of aspirated saline into the bronchi during simulated vomiting was measured at semi-lateral position angles of 0o to 90o in 10o increments. The difference in the vertical height between the mouth corner and the inferior border of the vocal cord was measured radiologically at each semi-lateral position in the three head-neck positions. RESULTS: Pulmonary aspiration was prevented at the ≥70o, ≥80o, and 90o semi-lateral positions in the neutral, simple extension, and sniffing positions, respectively. The mouth was lower than the vocal cord in the semi-lateral position in which aspiration was prevented. CONCLUSION: The lateral or excessive semi-lateral position was necessary to protect the lung from aspiration in the head-neck positions commonly used for tracheal intubation. Prevention of aspiration was difficult within clinically relevant semi-lateral positions.