We report two patients with chronic encephalomyelitis due to Borrelia burgdorferi in whom the definite diagnosis was delayed because of atypical clinical features. The first patient presented with chronic spastic paraparesis, slight ataxia and nystagmus of several years' duration. A tentative diagnosis of multiple sclerosis was made in spite of important abnormalities of the CSF biological characteristics. The second patient presented with an acute aphasia and a bilateral Babinski's sign. He was thought to suffer from benign herpetic meningoencephalitis. Several months later, as the patient experienced relapses with cerebellar and spinal cord involvement, falsely positive tests for syphilis were found and an antibiotic treatment was given. High protein content, low glucose levels, pleocytosis and oligoclonal bands were observed in all CSF samples, but the definite diagnosis was based on the detection of serum and CSF antibodies against B. burgdorferi.