The purpose of this prospective, randomized, single-blind study was to determine the anesthetic efficacy of lidocaine with epinephrine compared to lidocaine with epinephrine plus 0.5 M mannitol in maxillary lateral incisor infiltrations. Forty-one subjects randomly received 2 maxillary lateral infiltrations consisting of a 1.84-mL solution of 36.8 mg lidocaine with 18.4 μg epinephrine (control solution) and a 2.90- mL solution of 36.8 mg lidocaine with 18.4 μg epinephrine (1.84 mL) plus 0.5 M mannitol (1.06 mL) in 2 separate appointments spaced at least 1 week apart. The maxillary lateral incisor was blindly electric pulp-tested in 2-minute cycles for 60 minutes postinjection. No response from the subject to the maximum output (a reading of 80) of the pulp tester was used as the criterion for pulpal anesthesia. Total percent pulpal anesthesia was defined as the total of all pulpal anesthesia readings (at output of 80) over the 60-minute test period. Pain during solution deposition and postoperative pain were also measured. The results demonstrated that a 2.90-mL solution of 36.8 mg lidocaine with 18.4 μg epinephrine (1.84 mL) plus 0.5 M mannitol (1.06 mL) was not statistically significantly superior to a 1.84-mL solution of 36.8 mg lidocaine with 18.4 μg epinephrine. The pain of solution deposition was lower with the lidocaine/mannitol formulation. Postoperative pain was not statistically significantly different between the lidocaine/mannitol formulation and the lidocaine formulation without mannitol. We concluded that adding 0.5 M mannitol to a lidocaine with epinephrine formulation was not significantly more effective in achieving a greater percentage of total pulpal anesthesia (as defined in this study) than a lidocaine formulation without mannitol in the maxillary lateral incisor.