Low-grade inflammation has been found in patients with bipolar disorder (BD), but rarely assessed using leukocyte counts and findings are limited by lack of control for confounding factors. As a result, it is unclear whether BD per se is associated with peripheral inflammation. We pooled populations from two studies using similar longitudinal designs, including 300 blood samples from a total of 97 patients with BD and 133 blood samples from a total of 72 healthy control individuals (HC). Total leukocyte and neutrophil counts were measured together with interleukin (IL) - 6, IL-8, IL-18, tumor necrosis factor (TNF) - α and high sensitivity C-reactive protein (hsCRP). Adjusted for confounders, leukocyte counts were 23% higher and neutrophil counts were 30% higher in patients with BD compared with HC. There were no state-related differences in leukocyte or neutrophil counts. Lithium use, cigarette smoking as well as levels of IL-6, TNF-α and hsCRP were positively associated with leukocyte and neutrophil counts. Due to confounding issues it cannot be concluded that differences were related to bipolar disorder per se. Future studies are recommended to include leukocytes as markers of low-grade inflammation and to include relevant confounders in statistical analyses.