KEEPING SCORE: The Standard & Poor's 500 index rose 14 points, or 0.8 percent, to 1,830 as of noon Eastern Time. The Dow Jones industrial average gained 110 points, or 0.7 percent, to 16,137. The Nasdaq added 36 points, or 0.9 percent, to 4,036.
The S&P 500 dropped 2.7 percent last week, its worst weekly showing since January. The Nasdaq composite notched its third consecutive weekly decline.
SALES UP: Retail sales rose 1.1 percent in March, the best showing since September 2012, the Commerce Department reported. The government also revised February's figure to a 0.7 percent gain, more than double its previous estimate. Sales dropped in January and December.
"Retail sales was a really big number and very impressive," said Doug Cote, chief market strategist at ING U.S. Investment Management. "It bodes well for the market."
CITI: Citigroup reported a 2.5 percent increase in first-quarter profit. Both income and revenue beat Wall Street's expectations. The lender got a boost from improving results in its Citi Holdings unit, which is selling off assets such as mortgages that soured in the financial crisis. Citi's shares surged $1.91, or 4.2 percent, to $47.59.
POSITIVE PROGNOSIS: WebMD said it expects to report strong first-quarter results at the end of the month and anticipates reaching the upper end of its annual earnings and revenue projections. The health website operator's stock climbed $7.19, or 19.1 percent, to $44.88.
SECTOR WATCH: All 10 sectors in the S&P 500 index rose, led by energy and technology.
HEART STOPPER: Medical device maker Edwards Lifesciences rose the most of any stock in the S&P 500 index. A federal judge on Friday reaffirmed an earlier ruling that Medtronic's CoreValve system infringes on Edwards' patent for a replacement heart valve. Edwards soared $9.81, or 13.4 percent, to $82.78. Medtronic fell $1.27, or 2.2 percent, to 57.93.
STOCK SPOTLIGHT: Among the biggest risers in midday trading were F5 Networks, up $5.65, or 5.5 percent, to $108.60, and Micron Technology, which added 91 cents, or 4.3 percent, to $22.04. Decliners included Intuitive Surgical, which shed $7.27, or 1.7 percent, to $432.30. Gap slipped 70 cents, or 1.8 percent, to $37.70.
EARNINGS RADAR: Citi's solid quarter and the encouraging earnings outlook by WebMD were welcome surprises after an earnings miss last week by JPMorgan Chase that got the first-quarter earnings season off to a sluggish start. Analysts still expect first-quarter earnings for companies in the Standard & Poor's 500 to decline 1.6 percent from a year earlier, according to FactSet, a financial data provider. If profits do fall, it would be only the second quarterly drop in three years. Among the major companies due to report earnings this week are Johnson & Johnson, Google, General Electric and UnitedHealth.
BONDS AND COMMODITIES: In government bond trading, the yield on the 10-year Treasury note inched up to 2.65 percent from 2.63 percent late Friday. The price of crude oil rose 17 cents to $103.91 a barrel. Gold rose $7.60 to $1,326.60 an ounce.