Hypersexuality in Alzheimer's disease (AD) has been rarely investigated. Hypersexual behaviours should be classified as a sexual obsession and included in the "obsessive-compulsive disorder-like" spectrum. Hypersexuality has no proven treatment, although reports have described reductions of this behaviour using antiandrogen treatment, H2-receptor antagonists and antipsychotic drugs. Serotonin reuptake blockers seem to be effective in the treatment of sexual obsessions or compulsions and less on paraphilic disturbances. We present the case of a 54-year-old male patient with Alzheimer's disease with compulsive sexual behaviour as reported by his wife. A 18-FDG PET scan evidenced prevalent hypometabolism of the right hemisphere, congruent with neuropsychological evaluation. Donepezil, 10 mg per day, produced cognitive improvement but no effects on sexual behaviour. Therapy with SSRI was subsequently started (citalopram): after 60 days, the patient showed improvement in both the compulsive pursuit of sex acts and the level of frustration when refused.