To examine the possible influence of the blood-CSF barrier function on mathematical formulas for estimating intrathecal IgG production {IgG index, Tourtellotte’s formula [TOURT], Schuller and Sagar’s formula [Schull], and Reiber and Felgenhauer’s formula [IgG(loc)]}, we analysed serum and CSF samples from 105 healthy individuals, aged 18-88 years, and 816 consecutive patients without oligoclonal IgG bands (OCBs) in the CSF, aged 18-91 years. In healthy individuals there was no significant correlation between the CSF/serum (S) albumin ratio (i.e. blood-CSF barrier function) and the IgG index, a dimensionless quotient for determining intrathecal IgG production, and only a small correlation (r = –0.08, p < 0.05) in patients without OCBs. However, significant dependence on the blood-CSF barrier function was found, in both healthy individuals and patients without OCBs, for the formulas determining intrathecal IgG production in milligrams (per litre CSF or per day), i.e. for TOURT, SCHULL and IgG(loc). Therefore, we propose a new formula for the quantitative determination of intrathecal IgG production in mg/l, the IGGPROD, which is calculated as: CSF-IgG - [(0.51 X CSF-Alb X S-IgG)/ S-Alb]. IGGPROD is an empirical formula, based on findings in 105 healthy individuals 18-88 years of age, and does not depend on the blood CSF barrier function.