Mental Retardation (MR) is a spectrum of disorders characterized by deficits in intellectual and adaptive functioning which could result from a range of etiological factors. The prevalence of MR in the community is estimated to be about 1%. Presentation of psychiatric disorders in people with MR could be different from general population. Limited behavioral repertoire, cognitive capacity and communication difficulties can lead to atypical presentation of mental disorders. The prevalence figures for bipolar disorder in people with MR range from 0.9% to 4.8%. Considering the fact that vast majority of these patients are on antipsychotics, which can suppress some affective symptoms, it is possible that many cases with bipolar diagnosis are overlooked or are difficult to confirm. In these case reports, clinical presentation of two adult patients with bipolar disorder and their successful treatment with lithium is presented. We aim to raise awareness of bipolar disorder comorbidity, its atypical presentation and response to Lithium in this patient group.