Treatment of postoperative pain is often insufficient. It normally consists of systemic application of an analgesic drug or a regional technique of analgesia. Fentanyl-TTS may be a new approach for postoperative pain therapy. Fentanyl is incorporated into a transdermal system; after application to the skin continuous release of therapeutic doses is achieved for a period of 72 h. Serum peak levels are obtained 8-16 h after application; the serum half-life is about 16-21 h because of the dermal depot. Fentanyl-TTS was administered in several clinical studies for therapy of postoperative pain. The efficacy of this new form of application could be demonstrated. For the first 12 h the patients needed supplementary doses of analgesic drugs in the same range as the placebo groups because of the lag time of fentanyl-TTS. In the following 12 h the need for supplementary analgesics was significantly reduced. After removal of the patch, the need for analgesics was still reduced for 12 h. In 21 of 341 patients respiratory depression occurred under therapy with fentanyl-TTS; no respiratory depression was observed in the placebo groups. Thus, respiratory depression might occur in up to 9% of postoperative patients treated with fentanyl-TTS. Other adverse effects were nausea (62%), vomiting (26%), sedation (22%), urinary retention (11%), headache (5%), and dizziness (8%). Local reactions under the patch were erythema (39%) or pruritus (9%). These phenomena disappeared within a few hours. The pharmacokinetics of fentanyl-TTS have two major drawbacks: during the first 12-15 h the patients need supplementary analgesics, usually opioids.(ABSTRACT TRUNCATED AT 250 WORDS)