We designed a study to determine the relative analgesic potency of intramuscular heroin and morphine and to compare mood and side effects in 166 cancer patients with postoperative pain. Heroin was about twice as potent as morphine (95 per cent confidence limits, 1.6 to 2.6 times) in graded-dose, twin-crossover assays. Heroin provided an analgesic peak effect earlier than morphine (1.2 plus or minus 0.08 and 1.5 plus or minus 0.10 hours, respectively [mean plus or minus S.E.M.]). Doses with equal analgesic effects provided comparable improvements in various elements of mood, particularly feelings of peacefulness. Peak mood improvement occurred earlier after heroin than after morphine (1.2 plus or minus 0.10 and 1.8 plus or minus 0.13 hours, respectively). Both analgesia and mood improvement were less sustained after heroin at doses providing equal peak analgesic effects. The drugs shared the most common side effects, with no marked differences in their occurrence; sleepiness was the most frequent side effect after both drugs (46 per cent with each). Heroin has no apparent unique advantages or disadvantages for the relief pain in patients with cancer.