BACKGROUND AND PURPOSE: The characteristics and long-term outcome of Lyme neuroborreliosis (LNB) according to diagnostic certainty (definite vs. possible) are incompletely understood. METHODS: In this retrospective cohort study of adults with definite or possible LNB, clinical and microbiological characteristics and long-term outcome over 12 months were evaluated at a single medical center. Severity of acute disease and long-term outcome were assessed using a composite clinical score encompassing clinical findings and symptoms and by the probability of incomplete recovery. RESULTS: Amongst 311 adult patients enrolled from 2008 to 2017, 139 (44.7%) had definite LNB and 172 (55.3%) had possible LNB. The most frequent LNB manifestation was cranial neuropathy with or without meningitis (53.4%). Patients with definite LNB more often had Bannwarth syndrome (53.2% vs. 18.6%), more severe disease (6 points vs. 4 points), longer pre-treatment duration (median 21 days vs. 13.5 days), higher cerebrospinal fluid pleocytosis (median 139 × 10(6) /L vs. 11 × 10(6) /L) and higher rate of Borrelia seropositivity (84.2% vs. 68.6%) than those with possible LNB. Ceftriaxone was prescribed more often than oral doxycycline in definite LNB than in possible LNB (96.4% vs. 65.7%). Unfavorable outcomes decreased during follow-up, being higher in patients with more severe disease at enrollment and in those with possible LNB, but were not associated with antibiotic therapy. CONCLUSIONS: Early LNB, most often presenting as cranial neuropathy, was definitively diagnosed in less than half of cases. A better diagnostic approach is needed to confirm borrelial etiology. Ceftriaxone was not superior to doxycycline in the treatment of early LNB, regardless of diagnostic certainty. In this retrospective cohort study of 311 adults with Lyme neuroborreliosis (LNB), allocated according to diagnostic certainty, early LNB was definitively diagnosed in less than half of cases and the most frequent LNB manifestation was cranial neuropathy with or without meningitis. Patients with definite LNB more often had Bannwarth syndrome, more severe disease, longer pre-treatment duration, higher cerebrospinal fluid pleocytosis and higher rate of Borrelia seropositivity than those with possible LNB. A better diagnostic approach is needed to confirm borrelial etiology. Ceftriaxone was not superior to doxycycline in the treatment of early LNB, regardless of diagnostic certainty.