Several MacBook Pro users are reporting blank screens on the high-end Apple notebook after deploying the latest firmware upgrade.

The problem, described in Apple support forums, has left users with machines that still operate, but display no image on the internal display, or on external displays. The extent and cause of the problem is not certain. Apple was not immediately available for comment.

"The screen is completely black," a person called "Dannirid" said. "I can adjust the sound, there is NO start up chime, if I press CAPS LOCK it lights, and I can transfer files from it via Firewire to my old MacBook.

"For me, it sounds like the display is dead, but it's very strange since it only happened after I upgraded the firmware, and mistakenly closed the lid in the process."

Apple released the MacBook Pro EFI firmware update 1.5 last week. The update fixed "several issues to improve the stability of MacBook Pro computers," Apple said. No other details were given.

Apple distributes software and firmware updates automatically over the Internet through the update software preinstalled in Mac computers. A PC version of the application is also available for people who use Apple's iTunes music store.

Apple has been criticized for distributing its Safari Web browser to PCs users through its software update program. Critics say distributing a browser is a misuse of the application. Safari is a small player in the browser market, which is dominated by Microsoft Internet Explorer, with Mozilla Firefox a distant second.

The latest MacBook Pro troubles were first reported by the enthusiast site AppleInsider.

Apple has had firmware problems before within its MacBook line. The company in March released a fix for the MacBook Air after users reported that the thin and light notebook overheated because of problems with the internal fan.

Update 1.0 of the notebook's system management control "fine-tunes the speed and operation" of the fan, Apple said in releasing the software. The underperforming fan apparently caused the MacBook Air, which was released in January, to overheat and then lock up, leaving the machine unresponsive for 10 to 15 seconds or more.