The decision to aspirate a painful hip joint is often based on history, physical, and laboratory findings. The purpose of this study is to determine whether ultrasound is helpful in making decisions to aspirate a painful hip joint in children. The anterior capsular distance was determined in 10 children with painful or irritable hips. This anterior capsular distance is 2 to 4 mm in most hips. A side difference of 2 mm or more is considered a significant capsular distension. Based on these criteria and clinical parameters, 3 hips were aspirated, and 7 were observed without aspiration. None of the 7 hips subsequently required further diagnostic workup or treatment and all have had a satisfactory outcome. Ultrasound is a reliable method to detect capsular distension or joint effusion. It helps significantly in making the decision as to "when to aspirate" a painful or irritable hip joint.