Identifying the cause of polyarticular joint pain can be difficult because of the extensive differential diagnosis. A thorough history and a complete physical examination are essential. Six clinical factors are helpful in narrowing the possible causes: disease chronology, inflammation, distribution, extra-articular manifestations, disease course, and patient demographics. Patients with an inflammatory arthritis are more likely to have palpable synovitis and morning stiffness; if the condition is severe, they may have fever, weight loss, and fatigue. Viral infections, crystal-induced arthritis, and serum sickness reactions are common causes of acute, self-limited polyarthritis. Because chronic arthritides may present abruptly, they need to be considered in patients who present with acute polyarticular joint pain. Joint palpation can help to distinguish inflammatory synovitis from the bony hypertrophy and crepitus that typically occur with osteoarthritis. Extra-articular manifestations of rheumatologic disease may be helpful in arriving at a more specific diagnosis. Many classic rheumatologic laboratory tests are nonspecific. A complete blood count, urinalysis, and a metabolic panel may provide more useful diagnostic clues. Plain-film radiographs may demonstrate classic findings of specific rheumatologic diseases; however, radiographs can be normal or only show nonspecific changes early in the disease process.