A 50% nitrous oxide / 50% oxygen premix is administrated to the patient during the realization of liver biopsy in order to relax him and to reduce his pain during this painful medical treatment. This single investigator centre and national clinical trial is done blindly : half of patients will receive 50% nitrous oxide / 50% oxygen premix and half of them a placebo gas (mixture of 50% oxygen and 50% nitrogen). 94 patients will be included in this clinical protocol.

Inclusion Criteria:

• patient requiring a liver biopsy of focal lesion
• Age upper than 18 years
• OMS : 0 to 3
• patient consent form signed

Exclusion Criteria:

• patient unable to use Visual Analog Scale (VAS)
• OMS upper than 3
• coagulation disorder
• contraindication linked to the experimental product
• patient already included in another clinical trial uncompatible with this trial