BACKGROUND: Sarcoidosis is a multisystemic granulomatous disease of unknown etiology, while Lyme borreliosis is a multisystemic disorder caused by Borrelia burgdorferi. The purpose of this study is to evaluate the relationship between sarcoidosis and Lyme borreliosis in a region of Japan where Lyme borreliosis is endemic. METHODS: We determined the seroprevalence of anti-Borrelia burgdorferi antibodies as well as antibodies three Japanese Borrelia strains by enzyme-linked immunosorbent assay and dotblot assay using purified Borrelia-specific proteins in 46 patients with confirmed sarcoidosis and 150 controls (50 disease controls and 100 healthy controls) in Hokkaido, the affected region. RESULTS: Fifteen patients with sarcoidosis (32.6%) tested positive for Borrelia spirochete in both assays, compared with two disease controls (4.0%) and two healthy controls (2.0%). The seroprevalence of anti-Borrelia antibodies in patients with sarcoidosis was much higher in the affected region than in the region in our previous study were Lyme borreliosis is non-endemic. CONCLUSION: In a region where Lyme borreliosis is endemic, Borrelia infection may be partially associated with sarcoidosis.