Within this dissertation we discussed which differences exist in the three clinical phenotypes of rheumatoid arthritis (RA), namely undifferentiated arthritis (UA), autoantibody-negative RA (RA-) and autoantibody-positive RA (RA+), with a focus on clinical as well as patient-relevant outcomes. We also explored if treatment could be stratified on these clinical phenotypes from a clinical as well as an economic point of view.