A low-passage, Portuguese isolate of Borrelia lusitaniae, strain PotiB2, was inoculated into C3H/HeN mice and disease was monitored by histopathology at 8 weeks after spirochaete challenge. Ear, heart, bladder, femoro-tibial joint, brain and spinal cord were examined. B. lusitaniae strain PotiB2 (6 of 10 mice) and B. burgdorferi sensu stricto strain N40 (9 of 10 mice) induced similar lesions in the bladder of infected mice characterised as a multifocal, lymphoid, interstitial cystitis. Moreover, both B. lusitaniae PotiB2 and B. burdorferi N40 induced lesions in the heart of infected mice. The lesions induced by B. lusitaniae PotiB2 (2 of 10 mice) were characterised as a severe, necrotising endarteritis of the aorta, with a minimal, mixed inflammatory infiltrate (neutrophils, macrophages and lymphoid cells) extending into the adjacent myocardium. In contrast, B. burgdorferi N40 induced a periarteritis of the pulmonary artery (7 of 10 mice), with no involvement of the endothelium and more extensive inflammation and subsequent necrosis of the adjacent myocardium. This infiltrate was composed entirely of mononuclear cells, predominantly mature lymphocytes and plasma cells. No lesions were noted in the joints or central nervous system with inoculation of strains N40 or PotiB2, and co-inoculation of either strain with Ixodes ricinus salivary gland lysate did not affect the resulting pathology. Serology, examined 8 weeks after inoculation, indicated a different reactivity in mice infected with B. lusitaniae PotiB2 compared with B. burgdorferi N40. Immunoblot analysis demonstrated that mice with lesions resulting from infection with B. lusitaniae PotiB2 reacted only to the flagellin protein (41 kDa) or to flagellin and OspC, whereas mice infected with B. burgdorferi N40 reacted with multiple high and low mol. wt proteins, including flagellin, p93, p39, OspA, OspB and OspC. These results indicate that B. lusitaniae PotiB2 induced pathology similar to B. burgdorferi N40 when inoculated into susceptible mice. Moreover, these results establish the first animal model of disease with B. lusitaniae. This mouse model can be used to characterise the immunopathogenesis of B. lusitaniae infection and to delineate the proteins responsible for disease induction in susceptible mice.