A retrospective study was performed to determine the course of limb-length discrepancies occurring in patients with monoarticular and pauciarticular juvenile rheumatoid arthritis. Data were assessed on thirty-six patients followed to skeletal maturity, on fifteen patients who had not reached skeletal maturity but who had been followed for four years or more, and on forty-nine patients followed for three years or less. In seventy-two of the total of 100 patients the onset of the disease occurred before they were five years old, and ninety patients had involvement of the knee. All patients in whom the disease developed before the age of nine had overgrowth of the involved extremity, but that overgrowth never exceeded 3.0 centimeters. The major discrepancy developed within the first three or four years and either increased very slowly thereafter, remained level, or decreased. Of the thirty-six patients who were followed to skeletal maturity, in twenty-nine a discrepancy of 1.5 centimeters or more developed at some time during the period of assessment. Twelve of the thirty-six patients had diminution of the discrepancy to the extent that epiphyseal arrest was not required. Fifteen eventually had an epiphyseal arrest. Rapid premature closure of the epiphyseal growth plate occurred only in those patients in whom the disease developed after the age of nine years. This led to immediate shortening of the involved side and on occasion to marked limb-length discrepancies.