OBJECTIVE: To examine the effects of aspirin on the potential for oxidative modification of low density lipoprotein (LDL). DESIGN: Before and after trial. SETTING: University department of medicine within a district general hospital campus. PATIENTS: Ten healthy normolipidaemic volunteers drawn from laboratory and medical staff. INTERVENTIONS: Aspirin (enteric coated) 300 mg daily for two weeks. MAIN OUTCOME MEASURES: In vitro oxidation of LDL following ultraviolet C (UVC) irradiation with measurements made of malondialdehyde, conjugated dienes, and electrophoretic mobility. RESULTS: There was a significant decrease in malondialdehyde production from LDL modified by aspirin in vivo following exposure to UVC irradiation for 90 minutes, culminating in a 30% decrease by 240 minutes (mean (SD) 64.2 (9.12) v 89.6 (11.6) nmol/mg LDL protein, P = 0.029). These observations were borne out using LDL modified by aspirin in vitro. The UVC induced increase in relative electrophoretic mobility of LDL was also significantly reduced following aspirin treatment (mean (SD) 2.17 (0.16) v 2.66 (0.24), P = 0.012). CONCLUSIONS: Aspirin, both in vivo and in vitro, protects LDL against subsequent oxidative modification, providing an additional mechanism whereby aspirin may protect against atherosclerosis.