A 70-year-old man was admitted to the hospital because of polyarthritis.The patient had been well until 13 days earlier, when he began a 3-day course of treatment with cefixime because of a root-canal operation. Watery diarrhea developed four days later. He took diphenoxylate and then flew to Europe for a one-week vacation. Mild diarrhea continued. Five days before admission, while the patient was still on vacation, his left knee became stiff, painful, and swollen. The next day, these symptoms were worse, and chilliness and night sweats developed. He began a two-day course of treatment with azithromycin. Three days before admission, his right elbow and left thumb became painful and swollen, and he took 2400 mg of ibuprofen in 24 hours, with partial relief of the pain. On the day before admission, he discontinued the ibuprofen. That evening he returned to the United States and found that his temperature was 38.9°C. On the following day, his right knee and left ankle were swollen, and he came to this hospital.The patient was a physician. He had had Reiter's syndrome at the age of 22 years, with monarticular arthritis, urethritis, and conjunctivitis. Physical therapy was administered, and the symptoms subsided within six weeks, without recurrence. He had had mild aortic regurgitation for at least six years. A previous serologic test for syphilis was negative. He took nifedipine and lisinopril for hypertension of 10 years' duration. An operation for prostate cancer had been performed four and a half years before admission, without evidence of recurrence. There was no recent history of nausea, vomiting, abdominal pain or cramps, hematochezia, ocular symptoms, dysuria, rash, ingestion of unusual foods, or exposure to ticks.The temperature was 37.6°C, the pulse was 80, and the respirations were 20. The blood pressure was 130/80 mm Hg.On examination, the patient appeared well. No rash, conjunctival injection, or lymphadenopathy was found. A grade 2 aortic diastolic murmur was heard. The patient could flex his right elbow and extend it over a range of 20 degrees. The left first metacarpophalangeal and interphalangeal joints were swollen, erythematous, and painful. There were effusions in both knees, larger on the left side, with warmth but no erythema; a small effusion was present in the left ankle.The urine was minimally positive for protein. A test for rapid plasma reagin was negative. The prothrombin and partial-thromboplastin times were normal, as were the levels of urea nitrogen, creatinine, glucose, uric acid, calcium, phosphorus, magnesium, electrolytes, and lactate dehydrogenase. The results of other laboratory tests are shown in Tables 1 and 2. A diagnostic procedure was performed.T ABLE 1. H EMATOLOGIC L ABORATORY V ALUES .