Background: Low-grade inflammation and oxidative stress have been implicated as potential pathophysiological processes in bipolar disorder, but the underlying mechanism is unknown. Ferritin is a marker of iron stores and involved in redox processes and inflammation but its role in bipolar disorder is unclear. Methods: We investigated the possible association of increased plasma ferritin levels and bipolar disorder. We pooled two studies using similar longitudinal repeated measures designs and included 330 blood- and urinary samples from 95 patients with bipolar disorder across all affective states and 84 samples from 84 healthy control individuals. Plasma ferritin was measured along with multiple blood inflammatory markers and urinary markers of oxidatively generated damage to DNA and RNA. Results: Plasma ferritin levels, adjusting for multiple demographical- and lifestyle variables, did not differ between patients with bipolar disorder compared with healthy control individuals (b = 1.09, 95 % CI: 0.86 to 1.39, p = 0.49). Within patients with bipolar disorder ferritin levels were higher in a depressed state compared with euthymia (b = 1.12, 95 % CI: 1.01 to 1.24, p < 0.04), and ferritin levels were positively associated with Interleukin-18 blood levels and urinary levels of 8-oxodG. Limitations: Patients with bipolar disorder received medication which could potentially influence iron metabolism. Conclusion: Elevated ferritin levels in depressed patients with bipolar disorder may point to a role for iron metabolism in bipolar disorder pathophysiology, and potentially as a biomarker, linking low-grade inflammation with redox biology and the well-known increased risk of medical comorbidity and reduced life expectancy.