Despite its generalized use as drug therapy for multiple sclerosis (MS), the molecular mechanisms of action of interferon beta (IFNB) are still poorly understood. IFNB therapy is long-termed and clinical effects are not immediate, therefore reliable early biomarkers for IFNB activity should maintain a differential expression over time, but longitudinal studies at a transcriptional level have been rare. Microarrays were used to monitor 18 IFNB1b treated MS patients at four time points spanning a period of 1 year. Genes showing in the majority of patients the greatest and most consistent changes in their expression levels were studied. Interferon regulated genes were significantly overrepresented. Fifteen markers were differentially expressed during all three time points and followed a consistent time course pattern: EIF2AK2, IFI6, IFI44, IFI44L, IFIH1, IFIT1, IFIT2, IFIT3, ISG15, MX1, OASL, RSAD2, SN, XAF1 and the marker 238704_at. Except for the last one, these biomarkers were all formerly identified as being indicative for IFNB activity. Expression changes were both early detectable and long lasting and could thus be optimal biomarkers for IFNB activity in long-term studies. Other known biomarkers of IFNB activity were found to be differentially expressed just for certain periods after therapy onset: Interleukin-8 was a short lasting marker and changes in STAT1 were detected with delay.