Two male patients, aged 6 and 9 years, were suspected of having unilateral Perthes disease because of typical symptoms and clinical signs. Roentgenograms, including scintigraphy, were normal. Magnetic resonance imaging (MRI) snowed definite avascular necrosis in the affected hip. Roentgenograms subsequently also became positive for Perthes disease. These two cases document the advantages of MRI in early diagnosis of Perthes disease prior to development of typical radiologic changes.