Bipolar I disorder is characterized by recurrent manic and depressive episodes. Both, dopamine and oxidative stress, resulting from mitochondrial dysfunction, have been implicated in BD and volumetric and metabolic changes have been reported. Nevertheless, it remains unknown whether changes result from treatment or ongoing disease, or if they are present at first episode mania. We used R2* and quantitative susceptibility mapping (QSM) to assess differences in brain iron and myelination in a cohort of first episode mania patients compared to controls. Patients exhibited lower QSM values than controls in the caudate and amygdala, possibly linked to iron loss or hypermyelination.