Assets held in exchange-traded funds crossed $1 trillion for the first time, at the end of January, according to statistics kept by the National Stock Exchange.

Total assets held in exchange-traded funds and notes totaled a record of $1.02 trillion at month’s end, the Jersey City, N.J., exchange operator said.

That is an increase of 37% from a year earlier. At the end of January 2010, assets held in ETFs and ETNs totaled $744.7 billion, by its count.

The number of listed exchange-traded products based on indices totaled 1,121, compared to 947 a year earlier.

Net inflows for the month totaled $10.3 billion, with products tracking U.S. equities accounting for $10.1 billion of that.

Notional trading volume during January 2011 totaled $1.3 trillion, representing 29% of all U.S. equity trading volume.

The stats can be found here.

A product-by-product breakdown is here.

The NSX monthly statistics include shares of open-end exchange-traded products, encompassing U.S. listed shares of investment companies, grantor trusts, ETNs and commodity pools.

ETFs are roughly two decades old. They started to gain broad adherence after State Street Global Advisors introduced Sector SPDRs, pronounced Spiders, which followed the nine industry sectors found in the Standard & Poor’s 500. SPDR stands for Standard & Poor's Depositary Receipts.