IgG synthesis within the central nervous system is the expression of a local pathologic immune reaction. Its detection is of great importance in a large number of neurologic diseases, especially multiple sclerosis (MS). The results obtained from the study of 115 neurologic patients (52 patients with MS and 63 patients with other neurologic diseases) using three different mathematic formulas (Tourtellotte's formula, the index of Tibbling and coworkers, and Schuller and Sagar's formula) are presented and statistically analyzed. Despite their obvious differences, the results from these three formulas are in accordance in 76% of the patients observed. The clinical correlations of these formulas are discussed; none of them allows a specific diagnosis of MS. The formula of Schuller and Sagar seems the most sensitive, detecting local central nervous system IgG synthesis in 79% of patients with MS, compared with 54% of patients with MS utilizing the index of Tibbling and coworkers and 50% of patients with MS utilizing Tourtellotte's formula. No correlation was established between serum and cerebrospinal fluid albumin concentrations, nor between serum albumin and IgG levels in cerebrospinal fluid. Thus, no evidence exists to support the inclusion of variations in serum albumin levels in the calculation of intrathecal IgG synthesis. Furthermore, Schuller and Sagar's formula allows classification of neuroimmunologic diseases and the evaluation of antibody-specific activity (ASA) in locally produced IgG. The comparison of intrathecal IgG ASA with serum IgG ASA, which reflects local and general immunity, respectively, seems potentially important.