Background: Phelan-McDermid syndrome is caused by a deletion at chromosome 22q13.3, and results in a phenotype characterised by intellectual disability, features of autism, physical and mental health conditions. It is becoming increasingly recognised that bipolar disorder represents part of this phenotype. Materials and methods: This case study describes 2 patients with Phelan-McDermid syndrome presenting with bipolar mania at inpatient unit for adults with intellectual disability. Both patients presented with severe disturbance of their behaviour, at times exhibiting aggression, disinhibition and hypersexuality. Results: Despite treatment with a number of atypical antipsychotics and anticonvulsant mood stabilising agents, both patients showed the greatest improvement when started on lithium, and were successfully treated with this medication. Conclusions: This adds further support to the growing evidence of bipolar disorder contributing to the phenotype of Phelan-McDermid syndrome, and clinicians should have a low threshold for considering the use of lithium in these patients.