In a double-blind crossover study, morphine and oxycodone hydrochloride were administered to 20 patients who were experiencing severe cancer pain. The peroral doses were determined on the basis of patient-controlled intravenous titration. The assumed oral bioavailability ratios were 44% (group 1, first 10 patients) and 33% (group 2, last 10 patients) for morphine and 66% (group 1) and 50% (group 2) for oxycodone hydrochloride, respectively. However, the patients were able to readjust their oral dosings. Equal analgesia was achieved with both drugs, but the intravenous dose of oxycodone hydrochloride needed was 30% higher than that of morphine. The median calculated oral/intravenous ratios giving comparable analgesia were 0.31 for morphine and 0.70 for oxycodone hydrochloride. Morphine caused more nausea than oxycodone hydrochloride and hallucinations occurred only during morphine treatment. Otherwise, there were no major differences in the side effects between these two opioids.