OBJECTIVE Brain arteriovenous malformations (bAVMs) are rupture-prone tangles of blood vessels with direct shunting of blood flow between arterial and venous circulations. The molecular and/or cellular mechanisms contributing to bAVM pathogenesis and/or destabilization in sporadic lesions have remained elusive. Initial insights into AVM formation have been gained through models of genetic AVM syndromes. And while many studies have focused on endothelial cells, the contributions of other vascular cell types have yet to be systematically studied. Pericytes are multifunctional mural cells that regulate brain angiogenesis, blood-brain barrier integrity, and vascular stability. Here, the authors analyze the abundance of brain pericytes and their association with vascular changes in sporadic human AVMs. METHODS Tissues from bAVMs and from temporal lobe specimens from patients with medically intractable epilepsy (nonvascular lesion controls [NVLCs]) were resected. Immunofluorescent staining with confocal microscopy was performed to quantify pericytes (platelet-derived growth factor receptor-beta [PDGFR beta] and aminopeptidase N [CD13]) and extravascular hemoglobin. Iron-positive hemosiderin deposits were quantified with Prussian blue staining. Syngo iFlow post-image processing was used to measure nidal blood flow on preintervention angiograms. RESULTS Quantitative immunofluorescent analysis demonstrated a 68% reduction in the vascular pericyte number in bAVMs compared with the number in NVLCs (p < 0.01). Additional analysis demonstrated 52% and 50% reductions in the vascular surface area covered by CD13-and PDGFR beta-positive pericyte cell processes, respectively, in bAVMs (p < 0.01). Reductions in pericyte coverage were statistically significantly greater in bAVMs with prior rupture (p < 0.05). Unruptured bAVMs had increased microhemorrhage, as evidenced by a 15.5-fold increase in extravascular hemoglobin compared with levels in NVLCs (p < 0.01). Within unruptured bAVM specimens, extravascular hemoglobin correlated negatively with pericyte coverage (CD13: r = -0.93, p < 0.01; PDGFR beta: r = -0.87, p < 0.01). A similar negative correlation was observed with pericyte coverage and Prussian blue-positive hemosiderin deposits (CD13: r = -0.90, p < 0.01; PDGFR beta: r = -0.86, p < 0.01). Pericyte coverage positively correlated with the mean transit time of blood flow or the time that circulating blood spends within the bAVM nidus (CD13: r = 0.60, p < 0.05; PDGFR beta: r = 0.63, p < 0.05). A greater reduction in pericyte coverage is therefore associated with a reduced mean transit time or faster rate of blood flow through the bAVM nidus. No correlations were observed with time to peak flow within feeding arteries or draining veins. CONCLUSIONS Brain pericyte number and coverage are reduced in sporadic bAVMs and are lowest in cases with prior rupture. In unruptured bAVMs, pericyte reductions correlate with the severity of microhemorrhage. A loss of pericytes also correlates with a faster rate of blood flow through the bAVM nidus. This suggests that pericytes are associated with and may contribute to vascular fragility and hemodynamic changes in bAVMs. Future studies in animal models are needed to better characterize the role of pericytes in AVM pathogenesis.