In the course of performing culture isolation of Borrelia burgdorferi sensu lato for the diagnosis of Lyme borreliosis in Slovenia, we encountered nine patients who were infected with atypical strains. Molecular analyses of these strains suggested that they were more closely related to the North American tick isolate, strain 25015 (which belongs to the DN127 genomic group of B. burgdorferi sensu lato), than they were to the three species (B. burgdorferi sensu stricto, Borrelia garinii, and Borrelia afzelii) hitherto found to be associated with European Lyme borreliosis. Review of the case histories of these patients revealed some atypical clinical features and variability in clinical presentation. In this study, we present the clinical findings for these patients and discuss their significance for the diagnosis of Lyme borreliosis. The DN127 genomic group shares with B. burgdorferi sensu stricto the distinction of being present in both the Old and New Worlds.