In this study, we aimed to investigate the diurnal variations of malondialdehyde (NIDA), in patients with early breast cancer. Ten consecutive premenopausal patients with early stage breast cancer and 10 healthy volunteers were included. Blood samples were taken every 4 h for a period of 24 h. The peripheral blood cells were counted and the plasma MDA levels measured as an indicator of lipid peroxidation. The daily average MDA levels of the patients were significantly higher than that of the controls (2.7 +/- 0.2 mumol/ml versus 2.2 +/- 0.2 mumol/ml, P = 0.044, respectively). The plasma NIDA levels of the patients showed significant diurnal variations with the highest levels at 20:00 h and the lowest levels at 04:00 h. In the control group, the plasma MDA levels had no statistically significant diurnal variations. However, the MDA levels at 12:00 and 16:00 It were significantly higher than the levels at 04:00 h. The NIDA levels of the patients group were significantly correlated with WBC and neutrophils with a phase difference of 12 h (rho = 0.341, P = 0.012 and rho = 0.288, P = 0.035, respectively). Though there were weak Correlations between the NIDA levels and WBC and neutrophils in the control group (rho = 0.215, P = 0. 127 and rho = 0.249, P = 0.076, respectively), the phase difference was 8 h. In conclusion, the current cross-sectional study suggests that the phase differences in daily variations of lipid peroxidation may play a role in carcinogenesis. (C) 2003 International Society for Preventive Oncology. Published by Elsevier Science Ltd. All rights reserved.