Introduction and objective. Several Borrelia burgdorferi sensu lato species cause Lyme borreliosis throughout Europe and their geographic distribution may influence clinical manifestations of the disease. In Poland, Lyme borreliosis presents mainly with neurologic and cutaneous symptoms, while clinically overt arthritis is rare. The presented study investigates the prevalence of B. burgdorferi s.l. genospecies in a group of patients with different clinical forms and stages of Lyme borreliosis in north-east of Poland. This has not previously been studied. Material and methods. Preferential reactivity towards different B.burgdorferi s.l. species was investigated with a previously validated immunoblot assay in sera of 33 patients with disseminated Lyme borreliosis: 10 with neuroborreliosis, 6 with acrodermatitis chronica atrophicans and 17 with osteoarticular symptoms. Also typed were B.burgdorferi s.l. DNA isolated from the skin and synovial fluid of 7 patients with erythema migrans, acrodermatitis chronic atrophicans and arthritis. Results. Preferential reactivity was detected in 30 out of 33 serum samples. Of these, 25 reacted preferentially with B.afzelii, 3 with B. garinii and 2 with B. burgdorferi ss. B.burgdorferi DNA was isolated from all studied samples and typed as B.afzelii in 5. In a patient with acrodermatitis chronica atrophicans studied with both methods simultaneously, B.afzelii was identified by both genotyping and serotyping. Conclusions. Both methods gave consistent results, indicating B.afzelii as the main agent of all the clinical forms of the Lyme borreliosis in the study area.