ETFs with the highest net cash inflows include those that track safe-haven bonds, like the Vanguard Barclays Total Bond Fund (NYSEArca: BND), which added $1.2 billion, and the iShares Barclays 1-3 Year Treasury Bond Fund (NYSEArca: SHY), which gained $1.1 billion. Meanwhile, the riskier iShares MSCI Emerging Markets (NYSEArca: EEM) lost $1.5 billion, the small-cap iShares Russell 2000 (NYSEArca: IWM) dwindled by $1.0 billion and the speculative-grade iShares Barclays Capital High Yield Bond Fund (NYSEArca: JNK) dropped by $1.2 billion.

ETPs experienced inflows of about $4.2 billion for the month of May, making the total year-to-date cash inflow $63.1 billion.

As of the end of May, there were 1,465 U.S.-listed ETPs, compared to the 1,254 listed products for the same month last year, or a 17% increase in new funds.

For more information on ETF assets, visit our ETF performance reports category.

Max Chen contributed to this article.