Forty women undergoing elective Caesarean section under spinal anaesthesia using hyperbaric 0.5% bupivacaine were randomly allocated to receive either 0.5 mg or 1 mg intrathecal diamorphine. All women received diclofenac 100 mg at the end of surgery and morphine via a patient-controlled analgesia system. Oral analgesics were not used. Postoperative analgesia was more prolonged and more reliable in the 1-mg group. Mean time to first analgesia was 10.2 h in the 1-mg group and 6.9 h in the 0.5-mg group, and 45% in the 1-mg group used no morphine, compared with 10% in the 0.5-mg group. Mean morphine consumption over 24 h was 5.2 mg in the 1-mg group and 10.6 mg in the 0.5-mg group. Pain scores all tended to be lower in the 1-mg group but this was only significant at 4 h. There were no serious side-effects. Minor side-effects were common but well tolerated, and the incidence did not differ between the groups. If intrathecal diamorphine is used in combination with rectal diclofenac and without oral analgesia, then 1 mg provides superior analgesia to 0.5 mg without any worsening of the side-effects.