Background. A changed radiological picture has Bern observed in the initial x-rays of the unaffected proximal femoral growth plate in children with unilateral Perthes disease. The present study seeks to determine whether a thicker, horizontal growth plate in an etiological factor in Perthes disease.<br /> Material and methods. The research involved 173 children diagnosed with unilateral Perthes diseas, and a control group of 174 children aged 3-10. Alsberg's angle was measured to detect horizontal growth plate, while x-ray images were examined if the growth plate was thickened.<br /> Results. In 15 of the children with unilateral Perthes disease, the unaffected growth plate was horizontal (Alsberg's anle 840-900); in 8 children, the growth plate was thicker and radiolucent, mostly on its lateral side. No clinical symptoms accompanied these changes in the unaffected hip joint in the children we studied. In the control group, horizontal growth plate was noticed in the case of child (Alsberg's angle 840).<br /> Conclusions. Transient radiographic abnormalities of proximal femoral growth plate are risk factors for increased incidence of Perthes disease in children above 3 years of age. The reason for these transient abnormalities of femoral growth plates is probably temporary disturbance of the blood supply and mechanical strain before manifestation of the symptoms of Perthes disease.