The relative incidence of childhood-onset bipolar illness in the USA compared with that in Europe is controversial. We examined this issue in more than 500 out-patients (average age 42 years) with bipolar illness who reported age at onset of first episode, family history, and childhood physical or sexual abuse. Childhood or adolescent onset of bipolar illness was reported by 61% of those in the US cohort but by only 30% of those in The Netherlands or Germany. In the USA there was also twice the incidence of childhood adversity and genetic/familial risk for affective disorder. The findings deserve replication and further exploration.