Although bipolar disorder in adults has been extensively studied, early-onset forms of the disorder have received less attention. We review several lines of evidence indicating that pediatric- and early adolescent-onset bipolar disorder cases may prove the most useful for identifying susceptibility genes. Family studies have consistently found a higher rate of bipolar disorder among the relatives of early-onset bipolar disorder patients than in relatives of later-onset cases, which supports the notion of a larger genetic contribution to the early-onset cases. Comorbid pediatric bipolar disorder and attention-deficit/hyperactivity disorder (ADHD) may also define a familial subtype of ADHD or bipolar disorder that is strongly influenced by genetic factors and may, therefore, be useful in molecular genetic studies. There are no twin and adoption studies of pediatric bipolar disorder, but the heritability of this subtype is expected to be high given the results from family studies. Thus, pediatric- and early adolescent-onset bipolar disorder may represent a genetically loaded and homogeneous subtype of bipolar disorder, which, if used in genetic linkage and association studies, should increase power to detect risk loci and alleles.