Oxidized low-density lipoprotein (OxLDL) is a well-known risk marker for cardiovascular diseases. OxLDL has shown a variety of proatherogenic properties in experiments performed in vitro. In addition, immunological studies using monoclonal antibodies have revealed the occurrence of OxLDL in vivo in atherosclerotic lesions and patients' plasma specimens. Resent clinical studies have indicated the prospective significance of plasma OxLDL measurements; however, the behavior and metabolism of OxLDL in vivo is poorly understood. The mechanism by which LDL is oxidized is not clear, and the modified structures of OxLDL are not yet fully understood, partly because OxLDL is a mixture of heterogeneously modified particles. Here, I discuss the recent studies on oxidative modifications in OxLDL and its clinical and pathological features.