A 47-year-old woman with a solitary morphoea profunda on the flexor aspect of the left thigh is described. Clinically the lesion is characterized by a circumscribed, deeply indurated sclerotic plaque. The main histopathological features were sclerotic collagen changes and a striking accumulation of inflammatory cells in the deep dermis as well as in the subcutaneous tissue. Plasma cells and T- and B-lymphocytes were the predominant cellular components of the infiltrate. The patient had an elevated serum titre of antibodies to Borrelia burgdorferi. The patient was treated with penicillin, which brought about a remarkable improvement. These findings provide further support for the concept that in some cases morphoea profunda may be a manifestation of Borrelia infection.