This study deals with the monitoring of plasmatic levels of malondialdehyde, as the main indicator of oxidative damage to biomembranes. Malondialdehyde is determined by high-performance liquid chromatography (HPLC) after derivatization employing 2,4-dinitrophenylhydrazine. A clinical study involving 20 female patients suffering from ovarian and endometrial carcinomas has demonstrated elevated levels of malondialdehyde (10.1 +/- 1.1 microM), compared with the control group (7.5 +/- 2.7 microM). It has been further verified that surgical removal of the tumor leads to an additional increase in the plasmatic malondialdehyde content. This unfavourable situation can be effectively eliminated by administration of a single dose of vitamin E prior to surgery.