Background. The purpose of this study was to compare the inter-observer validity and intra-observer reliability of the Catterall and Herring classification systems used in assessing pathological changes in Perthes' disease. Material and methods. The material consisted in x-rays from 58 patients (12 girls and 46 boys, 63 hips, average age 6) treated conservatively or surgically. The x-rays were assessed by a board-certified orthopedist with 19 years of experience, an orthopedist just taking board exams with 5 years of experience, and a resident. For statistical analysis we used the weighted kappa coefficient and the percentage of agreement. Results. The majority of the cases were qualified to groups II and III, and the younger observers evaluated more severely. The least inter-observer validity in both classification systems occurred between the resident and the senior physician. When the material was divided into two age groups (younger and older than 6), the validity of the Catterall scheme was similar in both groups, while in the Herring classification there was greater inter-observer validity for the older children. The Herring system shows greater intra-observer reliability overall; in the two age groups, the Catterall system shows greater reliability in the younger children, and the Herring system in the older. Conclusions. The Catterall classification system shows less validity and reliability than the Herring system, especially in reference to children older than 6. The greatest difficulties in classification decisions occur between Catterall groups II and III.