Free radical-induced oxidative damage may be involved in the neurodegenerative process associated with Alzheimer's disease (AD). 8-Isoprostaglandin F(2alpha) (iPF(2alpha)-III) is an isoprostane derived from free radical-induced non-enzymatic oxidation of arachidonic acid. It is formed in vivo and is an indicator of lipid peroxidation. Measurements were made of iPF(2alpha)-III in the urine of patients with mild to moderate dementia associated with probable AD and compared to those in the urine of non-demented subjects, who were similar in age and gender. 2,3-Dinor thromboxane B(2) (dinor TXB(2)), a urinary metabolite of TXB(2) was also measured, and served as an indicator of the enzymatic transformation of a product of arachidonic acid. Enzyme linked immunoassays were used to measure iPF(2alpha)-III and dinor TXB(2) in the urine. The concentration of iPF(2alpha)-III was significantly elevated in urine of patients assessed to have mild to moderate dementia as compared to non-demented patients. The concentration of urinary dinor TXB(2) was also significantly elevated in the patients with dementia and probable AD as compared to the non-demented subjects. There was considerable overlap of values obtained for demented and non-demented patients for iPF(2alpha)-III and dinor TXB(2), respectively. The observed elevation of iPF(2alpha)-III suggests that patients with mild to moderate dementia associated with probable AD are experiencing significant oxidative stress. This finding is consistent with current data suggesting that oxidative stress may be occurring in patients with dementia and probable AD. The increase of dinor TXB(2) may indicate that enzymatic processes related to the metabolism of arachidonic acid-derived products are also increased in demented patients with probable AD.