In order to elucidate the possible spirochetal origin of morphea and lichen sclerosus et atrophicans (LSA), we investigated biopsy specimens from 13 patients with morphea and 13 patients with LSA. Four patients with acrodermatitis chronica atrophicans (ACA), three with erythema chronicum migrans (ECM), and 11 patients with other inflammatory dermatoses served as controls. Formalin-fixed, paraffin-embedded sections were stained by an avidin-biotin-immunoperoxidase method, using serum of a patient with ACA that contained IgG antibodies to Borrelia burgdorferi. As positive control substrate, formalin-fixed culture suspensions of B. burgdorferi strain B31 were used. They presented either as thin, mostly linear, but partially dotted, stained spirochetes or, in contrast, as thick, "swollen," heavily stained organisms. Identical structures could also be detected on histological sections of one patient with ECM, four patients with morphea, and six patients with LSA. These findings provide evidence for the spirochetal origin of both morphea and LSA.