The purpose of this pilot investigation was to determine the usefulness of analgesic onset time as a measure of topical anesthetic efficacy in patients with spontaneous toothache pain. Under blinded conditions, 20 patients with spontaneous toothache pain from an open tooth cavity were randomly assigned to receive either 20% benzocaine or placebo (80% polyethylene glycol). The medication was applied directly to the open cavity in a volume of 3 drops. Patients then depressed a stop watch when they initially experienced pain relief. Patients who did not obtain relief were assigned the maximum onset value of 600 seconds. The average analgesic onset time was 111.8 seconds in the benzocaine group and 289.0 seconds in the placebo group. In the benzocaine group, 90% of the patients reported some pain relief, while a surprisingly high 60% reported some pain relief in the placebo group. The results of this study suggest that in the spontaneous toothache pain model, analgesic onset time is a valuable measure of topical anesthetic efficacy. In addition, polyethylene glycol at a concentration of 80% may not be a totally inactive vehicle.