The aim of the study was to evaluate the advantages of magnetic resonance imaging (MRI) in determining subluxation in Legg-Calvé-Perthes (LCPD) disease. Twenty-six patients with unilateral LCPD received 33 MRI and plain radiographs. For each patient, acetabulum head index (AHI) was measured on both hips (affected and unaffected) in a blinded fashion. Measurements were made from the cortical bone margin on the plain radiograph and from the cartilaginous surfaces on MRI. On the unaffected side AHI was 92.8% on the plain radiograph and 85% on MRI. On the affected side, AHI was 87% on the plain radiograph and 77% on MRI. These differences were statistically significant. With regard to the unaffected side, the femoral head should be considered subluxated if AHI is less than 86% on the plain radiograph and less than 77% on MRI. On the affected side, in 14 cases the femoral head was well-contained on both the plain radiograph and MRI. In 11 patients the femoral head was subluxated both on the plain radiograph and on MRI. In 8 patients the femoral head was well-contained on the plain radiograph but subluxated on MRI. This was due to thickening of the cartilaginous portion of the femoral head, which was clearly seen on MRI. MRI appeared to be more sensitive in determining the subluxation of the femoral head during the active phase of LCPD.