In Switzerland 5-35% of Ixodes ricinus ticks are infested with Borrelia burgdorferi (B.b.). There is a high risk of transmission of this infectious agent from any tick bite and 4-5% of affected subjects subsequently contract evident Lyme borreliosis. However, both tick bite and erythema chronicum migrans are unreliable diagnostic pointers as they are not usually found in the history of Lyme borreliosis patients. Similarly, an increased titer of antibodies against B.b. is not evidence of Lyme borreliosis, since this increased titer is found in some 10% of the healthy population. Finally, even a negative antibody titer does not rule out the diagnosis. The special problems of diagnosis are investigated in 7 patients with articular Lyme borreliosis and 9 patients with CNS symptoms. Articular Lyme borreliosis must be diagnosed by elimination even where there is an increased titer of antibodies against B.b., since neither the clinical picture, nor laboratory analysis of the synovial fluid, nor histologic and radiologic investigations show specific findings. There is a wide spectrum of neurologic symptoms. Diagnosis is easiest in cases with typical clinical findings (meningopolyneuritis), but in all other cases it is still by elimination. Among laboratory tests, calculation of an antibody index has proven helpful. Nevertheless, it is not always possible to differentiate Lyme borreliosis from encephalomyelitis disseminata. Antibiotic treatment has been tried in doubtful cases.