There were more than 600,000 household service robots in use worldwide in 2003, and more than 4 million new units should join them over the next three years, according to a study released Wednesday by the United Nations Economic Commission for Europe. The report also said global sales of multipurpose industrial robots rose 19 percent last year to 81,800 units and orders for industrial robots in the first half of this year jumped 18 percent to a record level.

Despite the promise of robot helpers dreamed up in TV shows like 'The Jetsons,' useful service robots have been slow to arrive in homes. But in the past few years, advances have come in fields such as sensors, navigation software and processing power, making robots more viable.

Now consumers are beginning to snap up robot products such as the Roomba vacuum cleaner from iRobot. In addition, sales of robotic lawn mowers have started to take off, according to the report. There were 570,000 robot vacuum cleaners in homes at the end of 2003, and 37,000 robot lawn mowers, it stated.

Last year, there were 691,000 entertainment and leisure robots, the report said. These were mostly toys such as Sony's Aibo. The latest version of the robotic pooch is designed to play back music files and dance to the beat, as well as act as a video surveillance system. The market for entertainment and leisure robots will rise by about 2.5 million units by 2007, with new sales estimated at more than $4 billion, the report said.

Until very recently, robots have failed to live up to expectations, acknowledged Colin Angle, chief executive of iRobot Corp. of Burlington, Massachusetts, whose Roomba is a popular robovac and which also makes robots used by the U.S. military. 'Our biggest hurdle right now is skepticism,' Angle said. But 'we are just at a point where robots are becoming affordable ... and some of them can actually do real work.'

The worldwide stock of operational industrial robots will increase from about 800,000 units at the end of last year to 1 million at the end of 2007, according to the report.