The prognosis of patients with undifferentiated arthritis (UA) may vary from self-limited to severe destructive rheumatoid arthritis (RA). Based on the chance that these patients will develop RA and based on the safety profile of a course of methotrexate for 30-90 days, many clinicians consider using methotrexate in this patient category using the "n of 1" trial principle. During the last few years, more data on interventions in UA have become available that provide guidance in the prescription of drugs to UA patients.