BACKGROUND: Legg-Calve-Perthes disease is a juvenile form of osteonecrosis of the femoral head. The purpose of this study was to use serial perfusion magnetic resonance imaging (MRI) to determine the pattern and rate of revascularization of the femoral heads of patients with the active stage of Legg-Calve-Perthes disease. METHODS: We performed a prospective study of 29 patients (30 hips) with a mean age (and standard deviation) of 8.4 +/- 1.9 years who were diagnosed with Waldenstrom Stage-1 or 2 Legg-Calve-Perthes disease. All patients had >/=2 perfusion MRIs, and 21 patients (22 hips) had >/=3. Perfusion percentages of the femoral epiphyses were measured by 2 independent observers. Statistical analyses included calculation of the intraclass correlation coefficient, the paired t test, the Mann-Whitney U test, and the Kruskal-Wallis test. RESULTS: Initial perfusion MRIs showed the percent perfusion in the affected femoral heads to range from 5% to 70%. The average percent perfusion (and standard deviation) was 35% +/- 16% on the first MRI, which increased to 77% +/- 14% on the follow-up MRI acquired at an average of 10.5 +/- 2.9 months later (p < 0.01). Serial assessment showed a general pattern of revascularization starting from the periphery of the posterior, lateral, and medial aspects of the femoral epiphysis and converging toward the anterocentral region. The average rate of revascularization was 4.9% +/- 2.3% per month with a wide range among the patients (0.6% to 10.4% per month). CONCLUSIONS: Revascularization of the necrotic femoral head increased over time in a horseshoe pattern, starting from the posterior, lateral, and medial aspects of the femoral epiphysis. The rate of revascularization was highly variable among patients. LEVEL OF EVIDENCE: Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.