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When comparing the first full month of sales of Apple Mac OS 10.5 "Leopard" to the first full month of sales for Mac OS 10.4 "Tiger", dollar volume for Leopard was up 32.8% and unit volume up 20.5%, according to the NPD Group.

While the increases in dollar and unit volume can partially be attributed to the near-November launch - a key month for consumer shopping - and the growth in the number of Apple retail stores, NPD said the figures show that Apple has found the right formula for rolling out new versions of Mac OS X.

Leopard is the sixth version in as many years, a fact that Apple is quick to contrast with Windows, which went more than five years between new versions.

Apple began selling Leopard on 25 October, after a four-month delay due to the company's work on the iPhone. Despite its success, the launch has been marred with reports of bugs and security flaws, such as the unfortunate "blue screen of death" installation issue.