Many neurological diseases are accompanied by increased protein concentrations in the cerebrospinal fluid (CSF), described as a blood-CSF barrier dysfunction. The earlier interpretation as a "leakage" of the blood-CSF barrier for serum proteins could be revised by introduction of a "population variation coefficient" of the CSF/serum quotients for IgG, IgA and IgM (delta Q/Q) which is evaluated as a function of increasing albumin quotients (QAlb). The data presented here are based on specimens from 4380 neurological patients. These population variation coefficients were found to be constant over two orders of magnitude of normal and pathological CSF protein concentrations (QAlb = 1.6.10(-3)-150.10(-3)). This constancy indicates that there was no change in blood-CSF barrier related structures with respect to diffusion controlled protein transfer from blood into CSF and hence no change in molecular size dependent selectivity. The pathological increase of plasma protein concentrations in CSF in neurological diseases could also be explained quantitatively by a decrease of CSF flow rate due to its bifunctional influence on CSF protein concentration: reduced volume exchange, and as newly stated, increased molecular net flux into CSF without change of permeability coefficients. Again, on the basis of a changing CSF flow rate, the hyperbolic functions, which describe empirically the changing quotient ratios between proteins of different size (e.g. QIgG:QAlb) with increasing CSF protein content (QAlb) can likewise be derived from the laws of diffusion as the physiologically relevant description. The hyperbolic discrimination line between brain-derived and blood-derived protein fractions in CSF in the quotient diagrams for CSF diagnosis can be further improved on the basis of the large number of cases investigated. Other physiological and pathological aspects, such as high CSF protein values in the normal newborn, in spinal blockade, in meningeal inflammatory processes, CNS leukemia or polyradiculitis as well as animal species dependent variations can each be interpreted as due to a difference or change in the CSF flow rate.