The prevalence and frequency of comorbidity of possible bipolar disorder was examined with attention-deficit hyperactivity disorder (ADHD) in a nonreferred population of twins. Children and adolescents aged 7 to 18 years with a history of manic symptoms were identified from a population-based twin sample obtained from state birth records (n = 1610). The sample was enriched for ADHD; however, there was also a random control sample (n = 466), which allowed a look at the population prevalence of the disorder. Juveniles with threshold or below threshold manic episodes were further assessed for comorbidity with Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV; American Psychiatric Association, 1994) and population-defined ADHD subtypes (from latent class analysis) using Fisher's exact test. Nine juveniles who exhibited DSM-IV manic (n = 1), hypomanic (n = 2) or below threshold episodes (n = 6) were identified. The population prevalence of broadly defined mania in the random sample was 0.2%. The possible manic episodes showed significant comorbidity with population-defined severe combined and talkative ADHD subtypes. It can be concluded that there is a significant association of bipolar symptoms with two population-defined subtypes of ADHD. Episodes of possible bipolar disorders as defined by DSM-IV are uncommon in this nonreferred sample. Children and adolescents with ADHD appear to be only modestly at increased risk for bipolar disorders.