Four of 10 children with SLE kept under observation over the past nine years have developed avascular necrosis (AN) of the femoral head. The symptoms of AN are insidious and unpredictable and predate the radiologic diagnosis by weeks to months. In a comparison of these children with SLE, with and without AN, with a group of patients with nephrosis treated with corticosteroids and a group with glomerulonephritis treated with azathioprine, AN was related to the duration of daily steroid therapy rather than the total duration of steroid treatment; this was not true for azathioprine. The occurrence of AN in our patients while they were on alternate-day steroid therapy, or coincident with a relapse, suggests that its development is determined by underlying disease.