Seroma formation is the most prevalent postoperative sequela after breast cancer surgery. A total of 263 aspirations of seroma fluid in 42 patients were performed after mastectomy; cytokines were measured in 148 cases. The concentration of interleukin-1β (IL-1), interleukin-6 (IL-6), interleukin-8 (IL-8), interleukin-10 (IL-10), interleukin-12p70 (IL-12) and tumor necrosis factor α (TNF) were measured. The patients underwent 9.0 seroma aspirations on average (range 1-17) during an average of 30.7 days (range 7-72). The average cumulative seroma volume was 2056.1 mL (range 50-5130). In all samples, the maximal average concentrations of IL-6 (mean 10717 pg/mL, range 136-100000) and IL-8 (mean 7221 pg/mL, range 102-79828) were 55-200-fold above the serum/plasma levels of asymptomatic adults. In contrast, we observed levels similar to normal serum/plasma levels for IL-1 (mean 62.8 pg/mL, range 0-1226), IL-10 (mean 29.8 pg/mL, range 3.6-359), and lower-than-normal serum/plasma levels of TNF (mean 3.4 pg/mL, range 0-31.7) and IL-12 (mean 0.5 pg/mL, range 0-11.8). Patients with clinical infection had generally significant higher maximal IL-6 (p = 0.004) and IL-8 (0.019) than patients without clinical infection. However, most patients had no bacterial infection. None of the cytokines were associated with cumulative seroma volume, duration of seroma production or number of seroma aspirations. Seroma formation after mastectomy has a pro-inflammatory component, as indicated by the high levels of interleukin-6 and interleukin-8. However, these levels do not predict the course of seroma production.