Borrelia burgdorferi is the pathogen that causes Lyme disease. Patients frequently experience fatigue and malaise that can persist after antibiotic treatment. This study examined serological reactivity to B. burgdorferi in patients with chronic fatigue who were from a region in which Lyme disease is endemic. Blood and CSF were collected from patients without a history of infection due to B. burgdorferi (n = 12) and patients with persistent fatigue after antibiotic treatment of Lyme disease (n = 13). Serum and CSF were examined by ELISA for antibodies to B. burgdorferi, and routine studies of CSF were done. In the first group, one patient (8%) was seropositive; no patients had detectable antibodies in CSF. In the second group, nine patients (69%) were seropositive or borderline seropositive; seven (54%) had detectable antibodies in CSF. Unexplained abnormalities in CSF were noted in 42% and 31% of patients in each group, respectively. In this study positive serologies for Lyme disease were not found at a higher than expected rate for patients from a region of Lyme disease endemicity who had idiopathic chronic fatigue. Fatigued patients did show a surprisingly high rate of unexplained minor CSF abnormalities suggestive of CNS or meningeal dysfunction.