SAN FRANCISCO (Reuters) - U.S. venture capital firms raised $5.91 billion in the second quarter, but the total went to just 38 funds, underscoring the continuing trend of concentrating cash in the hands of top funds.

The latest results compared with 45 funds raising $2.61 billion a year ago, and 49 funds raising $5.26 billion in the first quarter, according to data from the National Venture Capital Association (NVCA) and Thomson Reuters.

The biggest funds raised in the second quarter included a $2.08 billion fund from New Enterprise Associates and a $1 billion fund from Institutional Venture Partners.

Kleiner Perkins Caufield & Byers raised a $525 million fund, and Mithril Capital, a new venture fund from investor and PayPal co-founder Peter Thiel, raised $402 million.

Meanwhile, many smaller firms are struggling to raise money, as investors turn sour on terms requiring their money being tied up for at least a decade and lower-than-expected returns often associated with venture.

Just ten new funds, rather than follow-on funds raised by an established venture-capital firm, were raised in the quarter. That is the fewest in over three years, the NVCA said.

(Reporting By Sarah McBride; editing by Jeffrey Benkoe)