In most cases, the onset of RA is between the ages of 20 and 50, but it can occur at any age, even in childhood (young children can have a form called juvenile RA). About 1% of Canadians are affected with RA, incidence increases with advancing age, in all races and ethnic groups. Women are about three times as likely as men to develop the disease. The predominant symptoms include pain, prominent swelling, and stiffness in the affected joint(s). The pain and stiffness, which are worse in the morning and after prolonged rest or immobility, last for at least 30 minutes upon rising, with increasing duration and intensity when the disease is more active. Increased pain and stiffness in the morning can last up to several hours when the disease is very active. There can also be systemic effects, such as fatigue, malaise, weakness, weight loss, anemia, and occasional fever. Rarely, RA can affect other organs of the body besides the joints (eyes, skin, nerves, lungs). As the disease progresses, additional joints become affected, with gradual loss of function. Long-term prognosis of RA is poor; approximately 80% of patients experience a reduction in functional capacity within 20 years of disease onset. Complications include progressive joint destruction, loss of mobility, and deformity. The continuous inflammation in the joints accounts for the damage or destruction of the joints. Life expectancy is reduced by between 5 and 10 years in individuals with RA due to both rheumatoid complications and an increase in non-specific causes of death. Cardiovascular disease is the most common increased cause of death in individuals with RA.

The causes of RA are largely unknown, but genetic factors likely play a role: the presence of certain genes (associated with the immune system) may increase the risk of developing RA and may predispose the individual to more severe disease. Environmental factors also play a role. Diagnosis is based on the overall pattern of the symptoms, physical examination, laboratory tests and x-rays; there is no single test for diagnosing the disease. However, a set of criteria developed by the American College of Rheumatology is typically used in clinical research. An individual is diagnosed with RA if they have at least four of the following seven criteria: morning stiffness in and around the joints lasting at least one hour before maximal improvement; soft tissue swelling (arthritis) of three or more joint areas as observed by a physician; arthritis of finger or wrist joints; symmetric swelling (arthritis in the same joint areas on both sides of the body); rheumatoid nodules; the presence of serum rheumatoid factor; and radiographic erosions in hand and/or wrist joints. The first four criteria must have been present for at least six weeks.