Shipments of handheld computers running Windows Mobile soared 64% in the first quarter, propelling worldwide shipments overall to 5.1 million units, a 39.7% increase over the same period a year ago, a market researcher says.

Windows Mobile licensees accounted for nearly 3.2 million shipments of personal digital devices in the first quarter, or more than 60% of total shipments, Gartner said.

"This strong growth was surprising because Windows Mobile 6 will launch in the second quarter, accompanied by the launch of many new and improved models," Gartner analyst Todd Kort said in a statement. "Normally, consumers and businesses have constrained their purchases when upgrades are imminent."

Research In Motion continued to lead the PDA market, but its product mix has shifted toward a different, but more lucrative, product category, smartphones. As a result, the BlackBerry Pearl accounted for 55% of RIM shipments in the first quarter. In the PDA market, RIM shipments were essentially flat, but Gartner predicts shipments would grow through the remainder of the year as the new 88xx and 83xx models replace the 87xx.

Mio Technology, the leading Microsoft licensee, shipped nearly 568,000 PDAs in the quarter, a 162.1% increase from a year ago. All the shipments were equipped with global positioning systems, which are popular in Europe and other regions.

The top five PDA vendors, according to market share, were RIM, 18.1%; Mio, 11.1%; Samsung, 8.8%; Sharp, 8.5%; and Nokia, 6.2%. Total shipments for all vendors in the quarter topped 5.1 million units, compared with 3.7 million a year ago.

Gartner defines a PDA as a data-centric handheld computer that weighs less than a pound and is primarily designed for use with both hands. The data-first devices may offer voice as a secondary capability.