To determine whether antiplatelet therapy following carotid bifurcation endarterectomy influences postoperative survival and whether signs of progression in the plaques harvested at the time of surgery might be a prognostic indicator, a controlled clinical trial was undertaken. During 1982 to 1985, 66 patients were recruited, operated on (carotid endarterectomy) and assigned, using the method of adaptive randomisation to the therapy group (n = 32) receiving 1.0 g acetylsalicylic acid (ASA) per day, starting day two prior to surgery, or to the control group (n = 34), which remained without antiplatelet medication. The plaques harvested at the time of surgery were processed using standard histopathological methods and examined "blindly" by light microscopy for signs of arterio-sclerotic progression. The final endpoint was patient survival. The last assessment of the participants survival status was done by June 1989. During follow-up, 20 patients died, six in the treatment group and 14 in the untreated group, the difference being statistically significant (P less than 0.013 Breslow, P less than 0.029 Mantel). In 27 instances the histo-pathological examination showed signs of progression. In this subgroup ASA yielded a significant prolongation of patient survival (P less than 0.017 Breslow, P less than 0.048 Mantel). In the remaining patients no signs of cellular infiltration were evident and no influence of ASA on patient survival was demonstrable (P less than 0.503 Breslow, P less than 0.390 Mantel).