Syphilis is a chronic granulomatous disease caused by infection with Treponema pallidum （TP）. We report a case of intractable oral mucosal ulcer of the lower right side of the lip that was suspected to be caused by syphilis.
A 27-year-old woman presented with an intractable mucosal ulcer. She had been receiving treatment for syphilis prescribed by a primary care physician and recovered from other symptoms （A few weeks before the first visit to our department, she had oral sex with her partner, who was suspected to have syphilis.）. Although the ulcer was about to heal, we performed a biopsy on the next day. However, no TP was detected. Two weeks later, the ulcer disappeared, and no recurrence has been detected as of 24 months after the first visit.
We thought that the underlying disease was primary syphilis caused by local reinfection with TP after resolution of secondary syphilis.
