ObjectiveTo confirm the inter-side difference of bone age evaluated with conventional ultrasound. MethodsIn this prospective study, patients aged between 3 and 15 years with referral for bone age evaluation from the pediatric outpatient and inpatient department in our general medical institute were enrolled from March to October 2021. Bone ages from left and right side were evaluated with ultrasound using a summed ossification ratio of radius, ulna, and femur in all patients. For comparison, radiographic bone ages were assessed with X-ray films of the left hand and wrist. ResultsA total of 192 patients were enrolled into the study, including 95 boys (10.5 years [3.2-14.6 years]) and 97 girls (8.7 years [3.7-15.4 years]). In the girls, the ossification ratio of the right radius was greater than the left (P = .030) and the ultrasonic bone age from the right side was more advanced than the left (P = .007). The differences of the left and right ultrasonic bone age were -0.66 to 0.61 years in the boys and -0.89 to 0.67 years in the girls. The differences of right ultrasonic bone age and radiographic bone age were -0.77 to 1.01 years in the boys and -1.12 to 1.14 years in the girls. The differences of left ultrasonic bone age and radiographic bone age were -0.73 to 0.91 years in the boys and -1.16 to 0.90 years in the girls. ConclusionsClinically important difference can be obtained from the left and right side when assessed bone age with the ossification ratio of bones by ultrasound, especially in girls. Both sides should be evaluated in clinical work.