OBJECTIVE: To investigate cerebrospinal fluid findings in relation to clinical and electrodiagnostic subtypes, severity, and outcome of Guillain-Barré syndrome based on 1500 patients in the International GBS Outcome Study. METHODS: Albuminocytological dissociation was defined as an increased protein level (>0.45 g/L) in absence of elevated white cell count (<50 cells/µL). We excluded 124 (8%) patients due to other diagnoses, protocol violation or insufficient data. CSF was examined in 1231 patients (89%). RESULTS: In 846 (70%) patients CSF examination showed albuminocytological dissociation, which increased with time from weakness onset: ≤4 days 57%, >4 days 84%. High CSF protein levels were associated with a demyelinating subtype, proximal or global muscle weakness and a reduced likelihood of being able to run at week 2 (OR: 0.42 (95% CI 0.25-0.70; p = 0.001) and week 4 (OR: 0.44 (95% CI 0.27-0.72; p = 0.001). Patients with the Miller Fisher syndrome, distal predominant weakness and normal or equivocal nerve conduction studies were more likely to have lower CSF protein levels. CSF cell count was <5 cells/µL in 1005 patients (83%), 5-49 cells/µL in 200 patients (16%) and ≥50 cells/µL in 13 patients (1%). INTERPRETATION: Albuminocytological dissociation is a common finding in Guillain-Barré syndrome, but normal protein levels do not exclude this diagnosis. High CSF protein level is associated with an early severe disease course and a demyelinating subtype. Elevated CSF cell count, rarely ≥50 cells/µL, is compatible with GBS after thorough exclusion of alternative diagnoses. CLASSIFICATION OF EVIDENCE: This study provides Class IV evidence that CSF albuminocytologic dissociation (defined by the Brighton Collaboration) is common in patients with GBS.