Sweet's syndrome with arthritis in an 8-month-old boy.

Sweet's syndrome was diagnosed in a 4-month-old boy. He was successfully treated with systemic corticosteroids. At the age of 8 months, he developed acute arthritis in his right knee. The synovial fluid was analyzed and revealed a very high neutrophil count and neutrophil activation with a detectable level of intraarticular granulocyte-monocyte colony stimulating factor (GM-CSF). Prednisone injection into the knee led to dramatic improvement. No recurrence occurred. Although arthritis and/or arthralgia are common features in adult patients with Sweet's syndrome, this is the first reported case of Sweet's arthritis in a child.