Background Although (hypo)manic symptoms are common in adolescence, transition to adult bipolar disorder is infrequent. Aims To examine whether the risk of transition to bipolar disorder is conditional on the extent of persistence of subthreshold affective phenotypes. Method In a 10-year prospective community cohort study of 3021 adolescents and young adults, the association between persistence of affective symptoms over 3 years and the 10-year clinical outcomes of incident DSM-IV (hypo)manic episodes and incident use of mental healthcare was assessed. Results Transition to clinical outcome was associated with persistence of symptoms in a dose-dependent manner. Around 30-40% of clinical outcomes could be traced to prior persistence of affective symptoms. Conclusions In a substantial proportion of individuals, onset of clinical bipolar disorder may be seen as the poor outcome of a developmentally common and usually transitory non-clinical bipolar phenotype.