Combination chemotherapy with a regimen of methotrexate, vinblastine, doxorubicin, and cisplatin (M-VAC) has produced long-term survival times in a significant proportion of patients with advanced nodal and metastatic urothelial tumors. The availability of the colony-stimulating factors has spurred interest in studies evaluating escalated dosing schedules of M-VAC in an attempt to improve major response rates and overall survival. More frequent use of the colony-stimulating factors, however, in this and other settings may be associated with unrecognized side effects. The authors report a case of arterial thrombosis in a 69-year-old man receiving recombinant granulocyte colony-stimulating factor (rhG-CSF) and escalated-dose M-VAC for treatment of a transitional cell carcinoma of the bladder. Incidents of venous thrombosis have been reported previously with the use of colony-stimulating factors, but, to the knowledge of the authors, this case represents the first report of an arterial thrombosis occurring in a patient receiving rhG-CSF.