A group of adult patients with acute nontraumatic monoarticular arthritis was studied. Using only the traditional clinical tools of history, physical examination, joint x-ray films, and synovial fluid culture, Gram's stain, and examination for crystals, 74% of the diagnoses made were achieved either immediately or within two to three days. Overall, 64% of the cases were diagnosed during follow-up. Other tests were not found to be helpful in the acute situation and should be utilized only if the tests already mentioned fail to yield a diagnosis. The prognosis of patients with acute monoarthritis that remains undiagnosed after thorough evaluation is generally good.