OBJECTIVES To assess factors associated with health-related quality of life (HRQoL) in MS patients after three natalizumab infusions. METHODS MS patients, newly enrolled in the manufacturer's restricted drug distribution program and participating in a longitudinal study of their experiences with natalizumab, complete assessments prior to treatment initiation and after the 3rd, 6th and 12th infusion. HRQoL was assessed with the SF-12v2 where higher physical and mental component summary (PCS and MCS) scores indicated improvement in HRQoL. Univariate, followed by multivariate, linear regression was used to identify factors associated with HRQoL after three natalizumab infusions. The factors used in this analysis include age, clinical characteristics (disease duration, number of prior MS treatments) and baseline (BL) disease severity measures (disease step (DS), functional status, fatigue score, cognitive functioning (CF) score). RESULTS At the time of this analysis 504 patients had completed BL and 3rd assessments. Mean age was 46.08±10.93, 78% of patients were female, and mean disease duration was 9.64±8.33 years. PCS and MCS scores improved significantly from baseline (PCS: 34.03 versus 36.02; p<0.001) and MCS: 43.17 versus 47.22; p<0.001). Multivariate analysis indicated that higher BL PCS scores (p<0.001), lower BL DS scores (p<0.001) and younger age (p=0.002) were associated with improvement in PCS scores after three natalizumab infusions. Similarly, higher BL MCS (p<0.001) and BL CF scores (p=0.001) were associated with improvement in MCS scores after the 3rd infusion. CONCLUSIONS Factors associated with improvement in PCS scores after the 3rd infusion were younger age, higher BL PCS scores, and lower BL DS scores (better ambulation) while those associated with improvements in MCS scores were higher BL MCS scores and higher BL CF scores. These results suggest that initiating natalizumab in younger patients having lower disability and better baseline HRQoL results in greater improvements in HRQoL after three natalizumab infusions.