Use of agomelatine (I) or its hydrates, crystalline forms and acid or base addition salts, to obtain a drug to treat Smith Magenis syndrome, is claimed. An independent claim is included for a composition comprising (I) optionally in combination with excipients. ACTIVITY : Antidepressant Hypnotic Cardiovascular-Gen. Anorectic. MECHANISM OF ACTION : 5-Hydroxytryptamine-2C antagonist Melatoninergic receptor agonist.