OBJECTIVE: Transmission of the brain-derived neurotrophic factor (BDNF) Val66 allele in children with a prepubertal and early adolescent bipolar disorder phenotype was examined. METHOD: The prepubertal and early adolescent bipolar disorder phenotype was defined as current DSM-IV bipolar I disorder (manic or mixed phase) with at least one cardinal mania criterion (i.e., euphoria and/or grandiosity) to ensure differentiation from attention deficit hyperactivity disorder. Probands (mean age=10.7 years, SD=2.7) were obtained by consecutive new case ascertainment from designated pediatric and psychiatric venues. Parents and probands were interviewed separately by research nurses who were blind to the probands’ diagnoses. Genotyping was done with TaqMan Assay-on-Demand. Analysis was done with the Family Based Association Test program. RESULTS: There were 53 complete, independent trios. The BDNF Val66 allele was preferentially transmitted (Family Based Association Test: χ2=6.0, df=1, p=0.014). CONCLUSIONS: This finding in child bipolar disorder is consistent with data for adults with bipolar disorder that show preferential transmission of the Val66 allele.