Multiple sclerosis (MS) is the most common autoimmune demyelinating disease of the central nervous system (CNS). Both genes and environment contribute to susceptibility to the disease, but their role is as yet poorly understood. Ten monozygotic (MZ) and 14 dizygotic (DZ) twin pairs either dis- or concordant for MS, obtained from The Finnish Twin Cohort, participated in the present study. The aim was to evaluate the role of genetic and environmental factors in the aetiology of MS in Finland. The data was compared to the original MS Finnish Twin Cohort study performed by Kinnunen and associates in 1988. The method used to recruit the twins was the same in both studies. The concordance of MS was assessed using probandwise concordance rates and tetrachoric correlations for MZ and DZ twin pairs. For estimating the contribution of genetic factors to susceptibility to MS, a polygenic multifactorial model was used and heritability estimated using structural equation models. In order to identify genes involved in MS pathogenesis, the gene expression profiles in peripheral blood mononuclear cells (PBMCs) obtained from MZ discordant pairs were analyzed by cDNA microarray technology detecting the expression of 8 300 genes. Expressions of the 6 most often upregulated genes were confirmed by relative quantitative reverse transcription polymeric chain reaction (QRT-PCR). Furthermore serum samples from 17 twin pairs and cerebrospinal fluid (CSF) samples from 6 twin pairs were tested by PCR specific for human herpes virus 6 (HHV6) and with reverse transcription PCR (RT-PCR) specific for enteroviruses (EVs). Immunogloubulin (Ig) G and M response against HHV6 in serum and CSF were analyzed using ELISA method. The pairwise concordance for MZ twins was 30% and for DZ twins 14%. The corresponding probandwise concordance rates were 46% and 25%. The genetic variance (heritability) was estimated to be 15.3% (95% Cl 0.0-77.6), the common environmental variance 73.7% (95% Cl 14.1-93.9) and the unique environmental variance 11.1% (95% Cl 2.3-30.0). We found six genes to be up-regulated by 40 % and one gene (G1P3) by 50 % in the twins with MS. The six most constantly expressed genes were G1P3, POU3F1, MX2, LAPTM5, HBA2 and HBB. No HHV6 DNA or EV RNA was found in any serum or CSF samples. Eighty eight percent of the twins with MS and 86% of the healthy twin siblings were positive for HHV6-specific IgG in serum. One twin with MS was also postitive for HHV6 specific-IgM in serum, whereas none of the healthy twins was IgM positive. All CSF samples were negative for HHV6 specific-IgG and -IgM in both groups. Based on these data the concordance of MS in DZ twins in Finland has increased during the past two decades, whereas in MZ twins it has remained the same. This observation, together with the estimated low heritability, would suggest that the recently reported increase in MS incidence in Finland could be mainly due to environmental factors. We were not able to demonstrate, however, that EV or HHV6 infections could have had such a causative role. The six genes that were up-regulated at least two-fold in 40% of the twins, may be related to MS pathogenesis, beta-interferon (INF-â) treatment or an unknown virus infection.