The aim of the study was to examine diagnostic sensitivity and temporal course of intrathecal Borrelia burgdorferi (Bb) antibody production in acute Lyme neuroborreliosis (LNB). We recruited consecutive adult patients with LNB diagnosis based on strict selection criteria. Serum and cerebrospinal fluid (CSFs) were obtained, and clinical examination was performed pre-treatment, and 13 days and 4 months post-treatment. Pre-treatment positive Bb antibody index (AI) was detected in 34 of 43 (79%). All nine pre-treatment Bb AI negative patients, and 26 of 34 pre-treatment Bb AI positive patients reported symptom duration <6 weeks. Eight patients, all Bb AI positive, reported symptom duration of 6 weeks or longer. Consequently, pre-treatment diagnostic sensitivity of Bb AI was 74% when symptom duration was <6 weeks, and 100% when 6 weeks or longer. Three patients converted from negative to positive Bb AI status post-treatment. The six patients who were persistently Bb AI negative had lower CSF cell count and protein at presentation, when compared with the patients with positive Bb AI. In conclusion, the diagnostic sensitivity of Bb AI is suboptimal in acute early LNB. Repeated post-treatment Bb AI testing, to confirm or reject LNB diagnosis, is unreliable, as the majority of initial Bb AI negative patients remained negative at follow-up.