BackgroundSyphilis is a sexually transmitted disease caused by the spirochete Treponema pallidum. Recently, its incidence has been increasing worldwide. We encountered a young woman who presented with induration mimicking squamous cell carcinoma in the lower lip, without major medical conditions.Case presentationA 25-year-old Japanese woman presented with a 1-month history of a painless induration in her lower lip. Because squamous cell carcinoma was suspected, a preoperation work up was performed, including laboratory tests, an ultrasonographic examination, and a biopsy. The ultrasonography findings showed an oval-shaped 17 × 11 × 12 mm tumor-like lesion with heterogeneous internal echo and an indistinct border. A pressure test and color Doppler sonography revealed that the lesion was soft with a very abundant blood flow. These findings suggested the possibility of underlying inflammatory causes rather than a neoplastic tumor. Serology tests for syphilis, including the anti-Treponema pallidum antibody and reactive rapid plasma reagin tests, were positive. The biopsy revealed no malignancy. Finally, she was diagnosed as having primary syphilis and treated with amoxicillin for 28 days. The rapid plasma reagin value gradually decreased and the initial induration in her lower lip disappeared.ConclusionThis case highlights the need for prompt examinations for possible underlying infective causes, such as syphilis, when seeing a painless induration with ulcer in the lip. Ultrasonography was helpful in the differential diagnosis of a tumor-like lesion and should be included in addition to syphilis serology tests, such as anti-Treponema pallidum antibody and rapid plasma reagin tests.