Apolipoprotein E was measured in paired sera and cerebrospinal fluid samples from 483 neurological patients. The average apolipoprotein E concentration was 7.5 (+/- 3.1) mg/l in cerebrospinal fluid and 93.5 (+/- 29.7) mg/l in serum. Mean apolipoprotein B concentrations in 88 patients were 0.77 +/- 3.4 mg/l in cerebrospinal fluid and 1.06 +/- 0.31 g/l in serum. The apolipoprotein E concentration in cerebrospinal fluid was much greater than expected for passive diffusion from serum. By contrast to apolipoprotein B, there was no correlation between the apolipoprotein E cerebrospinal fluid/serum concentration quotient and the albumin concentration quotient. This suggests that apolipoprotein E in cerebrospinal fluid, unlike apolipoprotein B, is locally synthesized and that the use of a cerebrospinal fluid/serum concentration quotient is unnecessary. In a control group (n = 64) the mean cerebrospinal fluid apolipoprotein E value (+/- SD) was 5.9 (1.6) mg/l. Elevated cerebrospinal fluid apolipoprotein E concentrations were observed in acute (n = 22, 12.5 +/- 6.2 mg/l) and chronic inflammatory central nervous system diseases (n = 15, 10.4 +/- 2.4 mg/l).