This article addresses two questions about gadolinium-based contrast agents (GBCAs) and nephrogenic systemic fibrosis (NSF): "Why did it happen" and "What have we learned"? It reviews the events leading to the discovery of an association between NSF and GBCAs. Various factors are elucidated that contributed to the delay between the time when GBCA came into widespread clinical use and a link was made with NSF, including use in renal-compromised patients, high-dose magnetic resonance angiography (MRA), lack of documentation and adequate databases, policy and regulatory changes, and an absence of scientific evidence. The authors conclude that the overriding cause was lack of awareness. J. Magn. Reson. Imaging 2009;30:1236-1239. (c) 2009 Wiley-Liss, Inc.