Recent reports have shown association between CYP2D6 polymorphism and neuronal degenerative diseases such as Parkinson's disease. We investigated the association between this polymorphism and the risk for developing multiple sclerosis (MS). Leucocyte DNA from 118 MS patients and a control group of 200 unrelated healthy individuals was studied for the occurrence of 8 different CYP2D6 allelic variants by using allele-specific PCR amplification, XbaI and EcoRI RFLP analyses. The frequencies for these allelic variants in the MS and control groups were, respectively: CYP2D6wt 75.0% and 79.3%, CYP2D6A 0.4% and 1.3%, CYP2D6B 11.4% and 12.0%, CYP2D6C 4.2% and 2.0%, CYP2D6D 3.0% and 2.3%, CYP2D6L 0.8% and 0.3%, CYP2D6L2 5.1% and 3.0%. The frequencies of subjects with high CYP2D6 activity (those carrying two or more functional genes) were 77.1% and 73.5% in MS and control groups. The frequencies of subjects with absent CYP2D6 activity (those lacking functional genes) were 3.4% and 4.5% in MS and control groups, respectively. These results indicate that mutations at the CYP2D6 gene do not seem to be a factor in determining susceptibility to MS.