We describe a patient who had transient, acute synovitis of the knee following a brief diarrheal illness. The case history is noteworthy in regard to the striking monocytosis and low glucose level on synovial fluid analysis, and the unexpected isolation of echovirus type 1 1 from the synovial fluid. We believe this is the first documented instance of direct echovirus type 11 infection of a joint, causing synovitis.Case report. A 30-year-old senior medical resident experienced a 24-hour illness characterized by watery diarrhea, sore throat, fever to 38.5"C, sweats, myalgias, and headache. Within 48 hours of symptom onset and 24 hours after the diarrhea had subsided, swelling and pain rapidly developed in the left knee. He had no history of arthritis or of rheumatic symptoms, although he had undergone medial menisectomy and anterior cruciate resection of the left knee 3 years earlier. There were no subsequent knee complaints and he denied recent injury. His remaining medical history was unremarkable.He was examined by a rheumatologist within 12 hours of the onset of the knee swelling. The general physical examination findings were remarkable only for a temperature of 38°C. He had no rash, lymphaden-