Nineteen patients with pain from advanced cancer stabilised on oral 4-hourly aqueous morphine who were to be converted to twice daily controlled-release morphine tablets (MST Continus) completed this study. Patients were randomised to receive either their usual 4-hourly morphine dose or placebo with the first dose of MST tablets. There were no significant differences between the treatment groups in ratings of pain intensity, pain relief or side effects, or in any of the measured pharmacokinetic parameters. No patients in the placebo group experienced any breakthrough pain, and nursing staff were unable to identify which patients had received the active dose of morphine elixir. We conclude that there is no need for a loading dose when aqueous morphine is changed to MST tablets.