Experimental studies have suggested that unfractionated heparin (UH) has antiinflammatory properties. It is unknown whether UH also has these properties in patients with acute ischemic stroke. Within 12-24 h of treatment onset we measured the acute-phase response as reflected by the erythrocyte sedimentation rate (ESR) and total number of leukocytes (x10(9)/l) in 706 consecutive patients with acute ischemic stroke treated with full-dose UH (n=450), or 300 mg/day aspirin (n=256). Clinical outcome (Mathew scale) at hospital discharge and the effect of factors such as treatment (UH and aspirin), and acute phase response were assessed using multivariate analyses adjusted for baseline confounders and incident complications. Separate models were created for patients with lacunar and nonlacunar stroke. Whereas there were not differences at baseline between the two treatment groups, total leukocyte counts (8. 0+/-4.1 vs. 8.6+/-3.2, P<0.01) and ESR (21.7+/-20.9 vs. 25.2+/-22.9, P<0.05) were statistically significantly lower in patients treated with UH. This effect of UH was more accentuated in patients with nonlacunar stroke. Overall, leukocytes (7.2+/-2.3 vs. 8.4+/-4.0, P<0. 01), and ESR (15.7+/-17.2 vs. 24.3+/-22.2, P=0.0001) were lower in patients with complete early recovery and this effect was restricted to patients with nonlacunar stroke. Whereas baseline impairment, symptomatic bleeding and stroke recurrence were independent negative outcome predictors, the use of UH was positively associated with early recovery in all patients. This study shows that full-dose UH reduces the acute-phase reaction that follows ischemic stroke more effectively than aspirin. The prognostic implications of such effect seem more notable in patients with nonlacunar stroke.