Acute arthritis can first present as a symptom of dangerous and rapidly progressing disease. It is quite easy to differentiate between arthritis and periarthritis. More problematical is correct early differential diagnosis of the acute arthritis. Determining whether one, several or many joints are affected can narrow the diagnostic possibilities. Arthrocentesis and synovial fluid testing provide much information and should be done at initial evaluation if possible. The presence or absence of fever, rash, family history of joint disease and exposure to infective organisms can further direct diagnostic studies and treatment. In general, to avoid masking clues, drug therapy should be delayed for mild symptoms until diagnosis is complete. This article is designed mostly for primary care physicians, residents and includes author's original data and review of recommended reading.