Abe T, et al. – This study was undertaken to investigate the association between forearm muscle thickness (MT) by ultrasound and the muscle cross–sectional area (CSA) of the forearm obtained by magnetic resonance imaging (MRI). The obtained data demonstrate that forearm MT is closely correlated with MRI–measured forearm muscle CSA in young and middle–aged adults and demonstrate that ultrasound–measured forearm muscle thickness may be a useful variable for evaluating muscle CSA and function in the forearm.