Maternal floor infarction (MFI) is a poorly understood placental lesion reportedly associated with intrauterine growth restriction (IUGR) and recurrence. In this study of MFI and the related placental disorder, massive perivillous fibrin deposition (MFD), semiquantitative histologic criteria for these diagnoses are defined and rates of IUGR and recurrence are assessed. Pathologic slides of 80 placentas diagnosed as MFI or MFD at the Brigham and Women's Hospital (1989-99) were reviewed and reclassified as classic MFI, transmural MFD, borderline MFD, or neither MFI or MFD. The prevalence of IUGR was determined, and placental slides from additional pregnancies were reviewed to evaluate recurrence. Among 25 cases originally diagnosed as MFI, 5 (20%) were reclassified by study criteria as classic MFI, 9 (36%) as transmural or borderline MFD, and 11 (44%) as neither lesion. Among 55 cases originally diagnosed as MFD, 27 (49%) were reclassified as transmural or borderline MFD, 4 (7%) as classic MFI, and 24 (44%) as neither lesion. IUGR was identified in no case with classic MFI, in 31% of cases with transmural or borderline MFD (P = 0.02), and in 11% of cases with neither lesion. Recurrence was documented in 1 of 7 (14%) second- or third-trimester placentas. Possible recurrence was suggested in 3 of 6 (50%) first-trimester spontaneous abortion specimens. Classification of intraplacental fibrin is subjective and problematic; almost half of potential cases of MFI or MFD did not fulfill our study's diagnostic criteria. MFD may be more common and more strongly associated with IUGR than classic MFI. Recurrence of these lesions appears to be infrequent among second- and third-trimester placentas, but may be relatively common in first-trimester spontaneous abortions.