Bipolar disorder (BPD) is a heterogeneous disorder in terms of symptomatology, treatment response and course. In order to define more homogenous phenotypes subtyping of BPD according to the age at onset (AAO) is a promising approach. The problem of non-valid definitions of the threshold between early and late onset has recently been alleviated by the use admixture analysis. This statistical method suggests the existence of three - early, intermediate and late - onset bipolar sub-groups peaking at 17, 27 and 46 years, respectively (Bellivier et al., 2001). Research testing the validity of BPD subtypes according to AAO has recently been reviewed by Leboyer and colleagues (2005). The bulk of existing studies have supported early onset bipolar disorder as a distinct subtype. This subtype is characterized by the frequent presence of comorbid disorders, including anxiety disorders, high rates of suicidal attempts, substance abuse as well as a history of childhood disorders such as ADHD, oppositional defiant and conduct disorder. Early onset BPD patients also revealed greater severity of clinical manifestations during episodes, poor long term outcome and poor responsive to lithium treatment. The stronger familial aggregation of this BPD subtype suggests that the subdivision of BPD according to AAO may also be helpful in the search for genetic factors. Indeed, several studies found different candidate genes to be associated with the early and late onset subtypes. In a recent genome-wide scan for genes involved in BPD in families ascertained through early onset bipolar probands the 3p14 region showed the most significant linkage.