The physicochemical properties, pharmacology, pharmacokinetics, serum concentrations and clinical effects, adverse effects and contraindications, and dosage of transdermally administered fentanyl are described, and clinical studies evaluating the use of a transdermal fentanyl system in the treatment of postoperative pain and chronic cancer-associated pain are reviewed. After application of a transdermal system, fentanyl is absorbed into the skin beneath the patch, where a depot forms in the upper skin layers. Plasma fentanyl concentrations are barely detectable for about two hours after patch placement. Eight to 12 hours after patch placement, concentrations approximate those achieved with equivalent i.v. doses of fentanyl. Some studies comparing transdermally administered fentanyl with placebo in postoperative patients showed that the patients who received fentanyl required fewer supplementary analgesics and reported less pain than the patients who received placebo. However, the overall efficacy and safety of the transdermal fentanyl system for the treatment of postoperative pain have not been adequately evaluated. Studies of cancer patients showed that transdermally administered fentanyl appears to be effective in the management of chronic, cancer-related pain. Dermatological reactions to the fentanyl patch are generally transient and mild. Other adverse effects are those that are commonly associated with narcotic analgesics. The 25-micrograms/hr patch should be used for initial treatment in patients not previously treated with narcotics. The dosage may be gradually increased until effective analgesia is obtained. Although experience with the product is limited, transdermally administered fentanyl appears to be effective for the long-term management of cancer-related pain.