This study evaluated whether a single or double injection is required to achieve first molar anesthesia. Subjects received posterosuperior alveolar nerve blocks. Presence or absence of anesthesia was evaluated with carbon dioxide dry ice pulp sensitivity testing. This was followed by mesial infiltration and a second pulp testing. The posterosuperior alveolar nerve block alone usually provided effective first molar anesthesia. A small percentage of subjects required an additional mesial infiltration to obtain anesthesia.