Until recently, it was considered that Down's syndrome precluded a diagnosis of mania, or gave rise to an atypical presentation. There have been seven case reports of mania in people with Down's syndrome and all these cases are reviewed. The clinical features of mania are noted to be similar to those previously described in individuals with learning disabilities due to other causes. However, all reported cases are male and none has a family history of affective disorder. In two of the seven men reported, the illness followed a rapid cycling pattern. Hypothyroidism and monoamine biochemistry in people with Down's syndrome are discussed in the context of these atypical features.