The antimetabolite 6-azauridine blocks the de novo synthesis of pyrimidines and causes increased serum levels of several amino acids including homocystine. 6-Azauridine was withdrawn from clinical use for the treatment of psoriasis because of the occurrence of arterial and venous thromboembolic episodes in some psoriatic patients. Utilizing a standard animal model for the recognition of venous and arterial thrombosis, 6-azauridine was demonstrated in this study to cause thrombosis without producing homocystinemia when administered orally or intravenously.