Leopard has achieved Apple's best-ever sales results for an OS upgrade ever, according to researchers at the NPD Group.

The launch exceeded the success of Tiger, noted analyst Chris Swenson, citing NPD sales data which showed first month sales of Leopard were up 20.5 per cent over those of Tiger. Revenue was 32.8 per cent higher than Tiger's, the analysts said.

NPD separated sales of boxed copies of Leopard from those preinstalled on new Macs. Apple retail stores boosted sales, according to the analysts, as did Leopard's debut in the run-up to Christmas and Thanksgiving (in the US).

"It's clear that Apple has hit upon the right strategy for rolling out new versions of its OS," Swenson said.

Nearly a third (32.8 per cent) of all Leopard units tracked by NPD were the five-license Family Pack, a major bump from the 20.4 per cent of Tiger's sales. That increased average sale prices of Leopard to $144.30 in its first month, compared with Tiger's first-month average of $128.50.