Rising stocks and rock-bottom interest rates have given wealthy Americans a huge advantage: cheap loans they can use to finance their lifestyle and minimize their taxes.

Banks say their wealthy clients are borrowing more than ever, often using loans backed by their portfolios of stocks and bonds. Morgan Stanley’s wealth management clients have $ 68.1 billion in securities-based and other non-mortgage loans outstanding, more than doubling five years earlier. Bank of America Corp. said it has $ 62.4 billion in securities-based loans, dwarfing its book of home equity lines of credit.

The loans have special benefits beyond the flexible payment terms and low interest rates offered. They allow borrowers who need cash to avoid selling in a hot market. Startup founders can monetize their holdings without losing control of their companies. The super rich often use these loans as part of a “buy, borrow, die” strategy to avoid capital gains taxes.

The simply rich are also borrowing against their wallets. When Tom Anderson started at Merrill Lynch & Co. in Cedar Rapids, Iowa, in 2002, many of his fellow advisors had only one or two equity-based loans in their portfolio of businesses. Over the years, he encouraged more clients to borrow and noticed that his peers were doing the same. It is now common for advisers to large firms to have dozens of these loans outstanding, he said. Merrill Lynch is now part of Bank of America.

“You could buy a boat, you could go to Disney World, you could buy a company,” said Mr. Anderson, who now consults with banks about how to manage the risks associated with these loans. “The tax benefits are impressive.”