A 64-year-old woman presented with bullous and ulcerating lichen sclerosus et atrophicus (LSA) on the neck, trunk, genital and perigenital area and the extremities. Histology of lesional skin showed the typical manifestations of LSA; in one of the biopsies spirochaetes were detected by silver staining. Despite treatment with four courses of ceftriaxone with or without methylprednisone for up to 20 days, progression of LSA was only stopped for a maximum of 1 year. Spirochaetes were isolated from skin cultures obtained from enlarging LSA lesions. These spirochaetes were identified as Borrelia afzelii by sodium dodecyl sulphate--polyacrylamide gel electrophoresis and polymerase chain reaction (PCR) analyses. However, serology for B. burgdorferi sensu lato was repeatedly negative. After one further 28-day course of ceftriaxone the lesions stopped expanding and sclerosis of the skin was diminished. At this time cultures for spirochaetes and PCR of lesional skin for B. afzelii DNA remained negative. These findings suggest a pathogenetic role for B. afzelii in the development of LSA and a beneficial effect of appropriate antibiotic treatment.