OBJECTIVES: Irritation fibroma in the oral cavity causes atrophy or squamous epithelium thickening with respect to external injury-associated factors. However, ulcers do not occur in most cases. This study aimed to elucidate the mechanism by which ulcers do not form, focusing on the vascular network in the mucosal epithelium of irritation fibroma. METHODS: Immunostaining was performed using an enzyme antibody method with primary antibodies against CD31 and Ki-67 in 17 cases of irritation fibroma in the buccal mucosa. One section was taken at three points from the margin and three points from just above the lesion for measurement. The number of blood vessels in the superficial and deep lamina propria at the measurement site were determined, and the area per blood vessel was measured. RESULTS: The number and area of blood vessels in the superficial lamina propria just below the lesion epithelium were smaller than those in the margin. No difference was observed in the number and area of blood vessels in the deep lamina propria between the margins and lesions. CONCLUSIONS: Our results suggest that the vascular network in the deep lamina propria is maintained and compensates for the nutrient supply to the covering epithelium.