Alcoa Inc , the largest U.S. aluminum producer, reported a first-quarter profit that slightly beat estimates, but its revenue missed Wall Street's target and its shares dropped 3 percent in after-hours trading on Monday.

Revenue in the quarter rose 22 percent to $5.96 billion, helped by rising aluminum prices, but it lagged the Wall Street forecast of $6.08 billion.

What's really going to worry the market is top-line growth, said Alan Lancz, president of the Alan B. Lancz & Associates investment advisory firm in Toledo, Ohio.

They've really improved margins as much as they can, and if you don't get the top-line growth now or it doesn't meet expectations, that leaves the market vulnerable. (For a graphic on Alcoa's earnings click on

The company's shares fell 3.4 percent to $17.17 in after-hours trading on the New York Stock Exchange.

Alcoa's output of alumina, or aluminum oxide, used in the production of aluminum metal, dipped slightly in the first quarter to 4.0 million metric tons from 4.2 million in the fourth quarter of 2010.

Alcoa shares have risen nearly 40 percent in the past six months, and some Wall Street analysts suggested they were encountering profit taking.

Alcoa's had a big run, so I don't think you're going to see a lot of upside from this number, said Stephen Massocca, managing director at Wedbush Morgan in San Francisco.

Income from continuing operations, excluding special items, was 28 cents a share, topping analysts' expectation of 27 cents a share, according to Thomson Reuters I/B/E/S.

Net first-quarter earnings were $308 million, or 27 cents per share, compared with a net loss of $201 million, or 20 cents per share in the same quarter of 2010, the Pittsburgh-based company said.

Alcoa said the improvement was driven by higher realized prices for aluminum and its raw material, alumina. There was growing demand for aluminum products in major end markets such as packaging, automotive, transportation and industrial products.

The price of aluminum has climbed more than 30 percent in the past year, from around $1,990 per ton during the first quarter of 2010 to around $2,600 currently.

(Reporting by Steve James, editing by Bernard Orr)