Laser nephelometry (LN) is a rapid and very sensitive method for simultaneous determination of albumin, immunoglobulins, C3c and C4 in diluted serum and paired cerebrospinal fluid (CSF) samples. It is very useful in routine analyses. Determination of C3c and C4 covers classical as well as alternative pathways of complement activation. In CSF, they are mostly derived from and related to serum values. Under physiological conditions, the addition of intrathecal C4 synthesis is likely. The incidence of complement activation within CSF is also influenced by the method of choice (native molecules, activation products and complexes, inhibitors) and the mode of interpretation of results according to the functional state of the blood-brain barrier (BBB). Calculation of indexes and the modified Reiber's graph method are valid means of detection of complement activation within CSF. Complement activation within CSF was confirmed in 36% (111/302) of neurological patients examined; in 55% (48/87) of patients with inflammatory and demyelinating diseases, in 40% (37/94) of patients with CNS infections and complications, in 33% (4/12) of patients with motor neuron diseases, in 27% (11/40) of patients with spinal cord compression and sequelae, in 25% (8/32) of patients with neoplastic disease, and in 17% (6/37) of patients with cerebrovascular accidents.