Nano-scaled materials for diagnostic (nanomedicine) and electronics purposes are presently under development. Contrasting with the potential benefits of nanotechnology, the effects on human health are poorly understood. Nanomaterials are known to translocate into the circulation, and could thus directly affect vascular endothelial cells (ECs), causing vascular injury that might be responsible for the development of atherosclerosis. To explore the direct effects of nanomaterials on endothelial toxicity, human umbilical vein ECs were treated with 1-100 µg/ml of hydroxyl fullerene (C₆₀(OH)₂₄; mean diameter, 7.1 ±2.4 nm) for 24 h. C₆₀(OH)₂₄ induced cytotoxic morphological changes such as cytosolic vacuole formation and decreased cell density in a dose-dependent manner. Lactate dehydrogenase assay revealed that maximal dose C₆₀(OH)₂₄ (100 µg/ml) induced cytotoxic injury. Proliferation assay also showed that maximal dose C₆₀(OH)₂₄ inhibited ECs growth. C₆₀(OH)₂₄ did not seem to induce apoptosis, but caused the accumulation of polyubiquitylated proteins and facilitated autophagic cell death. Formation of autophagosomes was confirmed by Western blotting using a specific marker, light chain 3 antibody and electron microscopy. Chronic treatment with low dose C₆₀(OH)₂₄ (10 µg/ml, 8 days) inhibited cell attachment and delayed ECs growth. The present study examined for the first time the toxicity of water-soluble fullerene to ECs. While fullerene changed morphology in a dose-dependent manner, only maximal dose fullerene caused cytotoxic injury/death and inhibited cell growth. ECs death seemed to be caused by activation of ubiquitin-autophagic death pathways. Although exposure to nanomaterials appears to represent a risk for cardiovascular disorders, further in vivo validations are necessary.