A mild chronic encephalopathy may be the most common neurologic symptom in patients with late stage Lyme disease. The symptoms tend to be diffuse and nonspecific, and patients typically report memory loss, sleep disturbance, fatigue, and depression. Among patients with these symptoms, it is generally felt that those with abnormal cerebral spinal fluid (CSF) have a neurological basis to their illness. A comparison of Lyme patients, with and without abnormal CSF, revealed that only the abnormal CSF group had lower memory test scores than normal controls. However, most patients in both Lyme groups complained of memory loss and also reported significantly more symptoms of depression and fatigue than controls. Thus, while depressive symptoms may not be a factor in objective memory performance, they may indeed play a role in perceived memory loss. A survey of the neuropsychological literature suggests that active neurologic involvement, the psychological consequences of chronic illness, and possibly residual neurologic deficits from past infection with Lyme disease all may affect the patient's perception of cognitive dysfunction.