The pathological progression and prognosis of traumatic femur head necrosis (TFHN) after femoral neck fracture (FNF) in children and adolescent is relatively unknown and has never been specifically characterized. As we speculated, the prognosis in such population would be poor and characterized as the high risk of femoral head collapse, hip deformity and degeneration in a short term. This retrospective case series enrolled 64 children and adolescent with TFHN who treated with observational treatment from 2000.1 to 2018.1. The primary outcomes, the progression of femoral head collapse, hip deformity (Stulberg classification) and hip degeneration (Tönnis grade), and their prognostic factors were analysed. Sixty-four patients with a mean age of 13 years (6-16 years) were included. A total of 28 hips (44%) showed unsatisfactory outcome and 25 (39%) hips collapsed progressively during a mean follow-up of 48 months (24-203 months). Finally, 38 hips (59%) experienced hip deformity, 20 of them were Class IV/V. Thirty-four hips (53%) generally progressed to osteoarthritis, 14 of them were classified as Grades II/III. The location of the lesion and the presence of subluxation were found to be related to progression of collapse; however, the presence of subluxation was the only independent risk factor of severe hip deformity and degeneration. TFHN in children and adolescent is a rapidly progressing disease with a poor prognosis characterized by a high risk of femoral head collapse progression. If the subluxation emerged, collapsed cases showed increasingly tendency towards hip deformity and degeneration.© The Author(s) 2021. Published by Oxford University Press.