Objectives To investigate the influence of preservation methods and processes on the plasma interleukin-6 (IL-6) stability. Methods Lithium-heparin plasma was collected from female patients: 32 female patients with invasive breast neoplasms and 20 healthy females. Each sample was divided into three tubes. Samples were stored at different temperatures or at different times. The concentration of IL-6 was detected. Results IL-6 levels in patients were not altered significantly compared to the control group after storage at 4 degrees C or 25 degrees C for 12 h. However, IL-6 levels were significantly higher compared to controls (p<0.05) after storage at 25 degrees C for 48 h. IL-6 levels in patients with breast neoplasms were significantly higher compared to the control group (p<0.05) when stored at 4 degrees C after 12 h. IL-6 levels in patients with breast neoplasm increased more than 10-folds compared to the control group after only 2 h storage at 25 degrees C. Conclusions Concentrations of IL-6 in breast neoplasms samples significantly change under different storage conditions. Pretreatment needs to be standardized for blood sample handling procedure. Comparison of different storage conditions of IL-6 levels may not be reliable.