Supplemental, "as-needed," administration of an opioid is a common approach to the problem of breakthrough pain in cancer patients. Oral transmucosal fentanyl citrate (OTFC) is undergoing investigation as a new treatment for breakthrough pain. The primary purpose of the study was to demonstrate that a single-unit dose of OTFC can safely and effectively treat breakthrough pain. A secondary goal was to determine appropriate dosing guidelines. This was a multicenter, randomized, double-blind, dose-titration study in 62 adult cancer patients using transdermal fentanyl for persistent pain. Consenting patients provided 2 days of baseline data to evaluate the performance of their usual breakthrough pain medication. Patients then randomly received 200 microg or 400 microg OTFC in double-blind fashion. (Patients were always assigned, rather than randomized, to 200 microg if 400 microg represented > 20% of around-the-clock medication.) Pain intensity (PI), pain relief (PR), and global satisfaction scores were recorded. OTFC was then titrated until the patient received adequate PR for each episode using one OTFC unit. Orders to titrate up were ignored one third of the time to improve the blind. Two days of baseline data were compared with 2 days of OTFC data after titration identified an effective dose of OTFC. Most patients (76%) found a safe and effective dose of OTFC. There was no meaningful relationship between the around-the-clock opioid regimen and the effective dose of OTFC. In open-label comparisons, OTFC produced a faster onset of relief and a greater degree of PR than patients' usual breakthrough medication. Somnolence, nausea, and dizziness were the most common side effects associated with OTFC. Most patients find a single OTFC dosage that adequately treats breakthrough pain. The optimal dose is found by titration and is not predicted by around-the-clock dose of opioids.