* Nike rises after results, outlook
* Biotechs advance for eighth session
* Indexes rise: Dow and S&P 0.9 percent, Nasdaq 0.7 percent (Updates to close)
By Caroline Valetkevitch
March 20 (Reuters) - U.S. stocks rose on Friday, pushing the Nasdaq to a 15-year high and helping the S&P 500 snap a three-week string of losses, following a pullback in the dollar, upbeat results from Nike and further biotech gains.
Recent sharp gains in the U.S. dollar have increased worries about the currency's impact on the earnings of U.S. multinationals. S&P 500 earnings projections for the first quarter and for 2015 have fallen sharply since Jan. 1.
Among early reporters, Nike jumped 3.7 percent to $101.98 as the biggest boost to the Dow after it posted a quarterly profit that beat market estimates. The world's largest sportswear maker sold more higher-margin shoes and apparel but warned that the stronger dollar would take a toll on its current quarter.
The Nasdaq biotech index rose for an eighth straight session, gaining 7.4 percent since March 10. It climbed 0.5 percent on Friday, powered by a 9.8 percent climb in Biogen Idec to $475.98. volume was high. About 9.2 billion shares changed hands on U.S. exchanges, compared with the 6.6 billion average for the month to date, according to data from BATS Global Markets.
The company said its experimental drug became the first Alzheimer's treatment to significantly slow cognitive decline and reduce brain plaque in patients with early and mild forms of the disease, according to a small study.
The Nasdaq ended just 22 points from its record closing high, while the S&P 500 ended less than 10 points below its record close.
Largely behind this week's gains was a statement from the Federal Reserve on Wednesday which signaled a less aggressive approach to raising interest rates than investors had expected.
"The Federal Reserve's created a situation where there's very little alternative to equities, so the path of least resistance for stocks will be up for a period of time," said Robert Lutts, president, chief investment officer at Cabot Money Management in Salem, Massachusetts.
The Dow Jones industrial average rose 168.62 points, or 0.94 percent, to 18,127.65, and the S&P 500 gained 18.79 points, or 0.9 percent, to 2,108.06. The Nasdaq Composite added 34.04 points, or 0.68 percent, to 5,026.42, a 15-year high.
For the week, the Dow gained 2.1 percent while the S&P 500 rose 2.7 percent, both snapping a three-week run of losses. The Nasdaq ended up 3.2 percent.
Wall Street's fear gauge, the CBOE Volatility Index, was down 7.5 percent.
Stocks trimmed gains just ahead of the close, which marked the expiration of stock options, index options, index futures and single-stock futures, known as quadruple witching.
The dollar was off 1.5 percent against a basket of major currencies and registered its biggest weekly decline since 2011.
Tiffany & Co. shares lost 4 percent to $82.93 after the upscale jeweler said quarterly sales fell for the first time in five years and are expected to decline further in the current quarter, hurt by the strong dollar.
Advancing issues outnumbered declining ones on the NYSE by 2,452 to 632, for a 3.88-to-1 ratio on the upside; on the Nasdaq, 1,661 issues rose and 1,116 fell for a 1.49-to-1 ratio favoring advancers.
The benchmark S&P 500 index posted 86 new 52-week highs and no new lows; the Nasdaq Composite recorded 209 new highs and 31 new lows.
Volume was high. About 9.2 billion shares changed hands on U.S. exchanges, compared with the 6.6 billion daily average for the month to date, according to data from BATS Global Markets. (Editing by Bernadette Baum and James Dalgleish)