Background. Despite extensive research, it is still unknown whether a 40% increase in the anteversion angle value is a primary or a secondary change in Perthes' disease. The aim of the study was to evaluate the variability of the anteversion angle in children with unilateral Perthes disease treated operatively with varus and derotation osteotomy. Material and methods. 53 patients between 5 and 10 years were included in this study. The stage and type of necrosis were classified using radiological examination according to the Reiberg and Cattarall classification. The value of anteversion was determined using clinical and ultrasound examination in addition to directly during surgery. After the operation, the anteversion angle was 11 degrees . During a 6-year follow-up period, ultrasound examination was performed every 12 months. Angle values obtained through direct measurements were compared with the norms published by Lanz and Wachsmuth, and ultrasound measurements were compared with the norms for the age groups. Results. 47 patients were found to be in stage III of the disease and 6 were in stage II. During the period up to 36 months after surgery, the value of the anteversion angle gradually reached the same value as in a limb considered as "healthy". During the following 36 months these values remained unchanged. Conclusions. An increased value of the anteversion angle in children with Perthes' disease is observed in a limb considered as "healthy". An increased value of the anteversion angle in children with Perthes' disease is a secondary deformation caused by the arrest of the physiological decrease of its value during development.