Elevated blood levels of homocysteine represent an independent risk factor for premature arterial vascular disease and thrombosis. We investigated whether homocysteine could induce tissue factor (TF) procoagulant activity in cultured human endothelial cells. Homocysteine increased cellular TF activity in a timeand concentration-dependent manner. Low concentrations of homocysteine (0.1 to 0.6 mmol/L), similar to those found in the blood of patients with homocystinuria, enhanced TF activity by 25% to 100%. Other sulfur-containing amino acids (cystine, homocystine, cysteine, and methionine) induced less TF activity than did homocysteine; however, β-mercaptoethanol and dithiothreitol were more effective than homocysteine in increasing TF activity. Preincubation of homocysteine with a sulfhydryl inhibitor such as JV-ethylmaleimide prevented homocysteine induction of TF activity. A quantitative polymerase chain reaction method indicated that homocysteine increased TF mRNA in endothelial cells. These results indicate that an atherogenic amino acid, homocysteine, can initiate coagulation by the TF pathway through a mechanism involving the free thiol group of the amino acid and by TF gene transcription. These data support the hypothesis that perturbation of vascular coagulant mechanisms may contribute to the thrombotic tendency seen in patients with homocystinuria.