Chronic intervillositis of unknown etiology (CIUE) is a rare placental disease characterized by intervillous infiltration of maternal macrophages and associated with poor pregnancy outcomes and a high risk of recurrence in subsequent pregnancies. Its pathophysiology remains unclear and prognostic factors have not yet been established. In addition, clear relationships between the histologic extent of lesions and the severity of perinatal outcomes have not been demonstrated. Our objectives were to validate a CIUE classification system based on the gradation of macrophagic infiltration of the intervillous space, and to attempt to correlate these results with perinatal outcomes. For this multicenter retrospective study, 3 pathologists reviewed all cases diagnosed with "intervillositis" between 1997 and 2018. Confirmed CIUE cases were semiquantitatively graded based on the percentage of macrophagic infiltrate in the intervillous space: grade 1 (5% to 10%), grade 2 (10% to 50%), and grade 3 (>50%). Multiple pregnancies and pregnancies with medical follow-up completed outside of the study centers were excluded. In total, 122 cases of CIUE in 102 patients were included in the study. Microscopic classification based on one criterion was easy to perform, and interobserver correlation was good. Grade 3 infiltration was strongly associated with poor perinatal outcomes and fetal growth restriction (P