A double-blind study of patients selected at random compared the analgesic and adverse effects of intrathecal methadone (1 mg) with those of intrathecal morphine (0.5 and 1 mg). The study was conducted on 30 patients who underwent major orthopedic or urologic surgery. The intrathecal opioid was administered at the end of surgery, and assessments began 1 h thereafter and continued for 20 h. Pain measurements, supplementary analgesia requirements, and adverse effects were recorded. Intrathecal morphine (0.5 and 1 mg) provided effective and prolonged analgesia. Methadone, however, was unable to ensure the same degree of analgesia; consequently, the median pain scores were consistently higher following methadone than morphine (0.5 and 1 mg) (P less than 0.05). The time to the onset of discomfort severe enough to require supplemental morphine was longer after intrathecal morphine than that following methadone (24 and 29 h with morphine 0.5 and 1 mg; 6.5 h with methadone; P less than 0.05). Respiratory depression (increases PaCO2) was not associated with methadone and morphine 0.5 mg but was common following morphine 1 mg (P less than 0.05). Facial pruritus was unique to intrathecal morphine. Urinary retention requiring bladder catheterization was more frequent following morphine than methadone, although this was not statistically significant. Nausea and vomiting were common to all groups. Intrathecal morphine (0.5 and 1 mg) provides superior postoperative analgesia to 1 mg methadone. Various explanations for the observed differences between the drugs are discussed, including the possibility that the dose of methadone used in the subarachnoid space was inadequate and that a larger dose might have produced an effect equal to that of morphine.