Unilateral mastectomy is associated with moderate to severe postoperative pain. A new regional anaesthetic technique has been recently described, call the PEC block. This technique consists of injecting local anaesthetic between different muscles of the chest in order to relieve postoperative pain. The investigators aim to verify that this technique has a clinical relevant postoperative analgesic effect. For that purpose, the investigators will randomise 50 patients scheduled to undergo unilateral mastectomy into two groups: one group will receive a PEC block after the induction of the general anaesthesia under ultrasound guidance, while the other group will not have this regional procedure.

Inclusion Criteria:

• ASA physical status I-III
• Age ≥ 18
• Patients undergoing unilateral mastectomy

Exclusion Criteria:

• Contraindications to regional block
• Pregnancy
• Inability to give informed consent
• History of alcohol or drug dependence/abuse
• History of long term opioid intake or chronic pain disorder