Objective: An isolated dysfunction of the blood-CSF barrier is characterised by an abnormal elevation of the albumin CSF/serum concentration ratio (Qalb) wit hout any other pathological CSF findings. Although common in routine CSF analysi s, the clinical significance of an isolated barrier dysfunction frequently remai ns unclear. We examined neurological disorders associated with an isolated eleva tion of Qalb to identify possible determinants of blood-CSF barrier dysfunction . Methods: 367 patients (124 women, 243 men, median age 60. 0 years)out of 3873 patients receiving diagnostic lumbar puncture at the University Hospital of Ulm (Germany) showed an isolated dysfunction of the blood-CSF barrier. Clinical dat a as well as MRI findings of these patients were analysed. Results: Isolated bar rier dysfunction occurred most frequently ( > 30%) in Guillain-Barrésyndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), normal pressure hydrocephalus (NPH), lumbar spinal stenosis, and polyn- europathy (PNP). In patients who showed no other evidence of neurological dise ase, isolated barrier dysfunction was found in 14. 9%of cases. The extent of ba rrier dysfunction was most prominent in brain tumours, GBS, and CIDP. There was a significant correlation of Qalb with both weight and body mass index (BMI). Co nclusions: Although isolated barrier dysfunction may be found in a variety of ne urological diseases, it is especially frequent in GBS, CIDP, NPH, spinal canal s tenosis, and PNP. In these patients, disease-related mechanisms contributing to barrier dysfunction are likely. Moreover, barrier function seems to be influenc ed by disease-independent determinants like weight and BMI. Objective: An isolated dysfunction of the blood-CSF barrier is characterised by an abnormal elevation of the albumin CSF / serum concentration ratio (Qalb) wit hout any other pathological CSF findings. We examined neurological disorders associated with an isolated eleva tion of Qalb to identify possible determinants of blood-CSF barrier dysfunction. Methods: 367 patients (124 women, 243 men, median age 60. 0 years) out of 3873 patients receiving diagnostic lumbar puncture at the University Hospital of Ulm (Germany) showed an isolated dysfunction of the blood-CSF barrier. Results: Isolated bar rier dysfunction occurred most frequently ( > 30%) in Guillain-Barrésyndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), normal pressure hydrocephalus (NPH), lumbar spinal sten osis, and polyn- europathy (PNP). In patients who showed no other evidence of neurological disesese, isolated barrier dysfunction was found in 14.9% of cases. The extent of ba rrier dysfunction was most prominent in brain tumors, GBS, There was a significant correlation of Qalb with both weight and body mass index (BMI). Co nclusions: Although isolated barrier dysfunction may be found in a variety of ne urological diseases, it is especially frequent in GBS, CIDP, NPH, spinal canal s tenosis, and PNP. In these patients, disease-related mechanisms contributing to barrier dysfunction are likely. Moreover, barrier function seems to be affected ed ed by disease-independent determinants like weight and BMI.