Malignant transformation of cutaneous lichen planus is a rare event. We report a 34 year old Caucasian male who presented with an exophytic tumor on the right foreleg. The tumor gradually developed within previous areas of histologically proven hypertrophic lichen planus that had existed for about 10 years. However, the current histological examination of the excised tumor revealed highly differentiated squamous cell carcinoma with a depth of tumor invasion of 10 mm. At that time, neither sentinel lymph node biopsy nor further imaging diagnostics revealed evidence for metastatic spreading. Nevertheless, five months after surgery inguinal lymph node metastases were detected. Initial chemotherapy and inguinal lymph node dissection were unable to stop the spread of the tumor. One year later, parailiacal lymph node metastases were detected by computed tomography. Further cycles of chemotherapy resulted in significant reduction of the parailiacal tumor masses. This report indicates that the long-standing hypertrophic form of lichen planus seems to have a considerable propensity for malignant transformation, even in young patients.