To determine the importance of individual factors to thromboembolic stroke (TES) risk, the authors performed a hospital-based case-control study. Ninety cases (56 men and 34 women, ages 15 to 65) discharged from the hospital between January 1981 and December 1984 with a diagnosis of TES supported by computed tomography were matched to 174 control patients (106 men and 68 women). Data on potential risk factors were obtained from the medical record and telephone interview. Using multivariate analysis, three variables were significantly associated with TES risk: hypertension (odds ratio [OR] = 3.4; 95% confidence interval [CI] 1.9-6.0), diabetes (OR = 4.0; 95% CI 2.0-8.3), and smoking (OR = 2.0; 95% CI 1.2-3.6). The data were also analyzed using a direct risk assessment method. This analysis describes the risk in patients with any one factor compared with patients without any of the factors. The direct estimates of risk increased by 71% for hypertension (OR = 5.8), 28% for diabetes (OR = 5.1), and 90% for smoking (OR = 3.8). The authors conclude that hypertension, diabetes, and smoking are the major risk factors for TES in patients 65 years old or younger.