The Sarbanes-Oxley Act is an American law which greatly tightened many aspects of the regulation in the wake of accounting scandal such as that associated with the collapse of Enron.
The act had a wide range of provisions, such as increasing regulation of auditors, in an attempt to ensure their independence of the companies they audited. Many aspects of Sarbanes-Oxley regulations impose significant costs, such as the very strict rules on internal controls.
The expenses involved have lead many foreign companies to reconsider listing in US markets. In particular, many British companies have cancelled US listings.
While the consensus view is that Sarbanes-Oxley has lead New York to lose business to London, there is evidence that this may not have actually been the case, and that London's gains and New York's loss of market share are in fact the result of other factors.