The prognosis of patients with undifferentiated arthritis (UA) may vary from self-limited to severe destructive rheumatoid arthritis (RA). Because early aggressive treatment might offer an effective means to slow disease progression in RA, it is important to identify UA patients who will develop RA and treat them as early as possible. At the same time, inappropriate treatment of patients with a more benign disease course should be avoided. Here, an overview is given of the characteristics and numbers of patients with UA who evolve into RA. UA is defined as any arthritis that has the potential for a persistent course, without fulfilling the classification criteria for specific rheumatic disorders. To compare endpoints in the different databases, the 1987 ACR criteria for RA were used. In the nine databases employing a similar definition for undifferentiated arthritis, the proportion of patients with UA that evolved into RA within 1 year varied from 6% to 55%. These differences arise in large part from differences in the inclusion criteria and in the definitions used for UA and RA. The data from the various cohorts support a hypothesis that a considerable proportion of UA patients are actually patients with RA in a very early stage. Controlled intervention studies with early antirheumatic treatment in these patients are mandatory in order to provide further insight into the natural course of UA and to define a treatment strategy that will successfully slow or prevent disease progression.