Mechanisms of T lymphocyte trafficking in the brain remain unclear in MS. We hypothesized that MS is associated with increased CD4+ and CD8+ T lymphocyte trafficking across the BBB. To test this hypothesis, we calculated the frequency of PSGL-1+/CD4+ and PSGL-1+CD8+ or LFA-1+/CD4+/CD8+ T cells in the PBMC of 27 patients with a RR-MS (21 untreated and six IFN-beta-treated) and 18 HI. Next, we measured their ex vivo TR across resting and TNF-alpha-activated human BBB-derived hCMEC/D3 endothelial layers under static conditions. The frequency of PSGL-1+CD4+ T lymphocytes was significantly higher in treated or untreated MS patients than HI. Furthermore, resting hCMEC/D3 TR of CD4+ lymphocytes (purified or in PBMC) from treated or untreated MS patients were significantly higher than those of HI and associated with significant enrichments of CD4+PSGL+ or CD4+PSGL-1+CD45RO+ T cells in their transmigrating fractions. The TR of CD4+ and CD8+ from MS patients across TNF-alpha-activated hCMEC/D3 were also significantly higher than that observed in HI. Resting hCMEC/D3 transmigration was blocked significantly by anti-PSGL-1/anti-LFA-1 in all groups, and anti-VLA-4 inhibited transmigration of MS T cells specifically. Purified PSGL-1-negative CD4+ lymphocytes transmigrated resting hCMEC/D3 with <10% of transmigrating cells re-expressing PSGL-1, suggesting PSGL-1-independent transmigration mechanisms. The frequency of PSGL-1 was unchanged in CD8+ cells from MS patients, whereas CD8+LFA-1(high) were reduced significantly in IFN-beta-treated patients specifically. Collectively, MS is associated with an expanding pool of PSGL-1+CD4+ T lymphocytes able to transmigrate the BBB endothelium in vitro and possibly contributing to brain pathology.