OS 10.5.8 imac
Suddenly my primary button on my mouse will only scroll up.
It was okay this morning.
I checked system prefs and the scrolling is set at vertical only. What has happened? Could it be a defective mouse?

jaybass

Have you tried to see if a reboot fixes the problem?

If it's an Apple Mighty Mouse it's a common problem, happens all the time, nothing to do with system preferences, indeed this mouse on this iMac is now doing exactly the same thing, which was predictable, regrettably.

There are dozens of internet threads about cleaning the Mighty Mouse scrolling button (have a go!) but eventually you'll have to replace it.

HTH

Yes I did restart but no joy.

And to Mr Bensheim I will try to clean it but first I'll phone apple as this imac is only 8 months old.
Maybe they will give me a new one.

jaybass

I was amiss above by not asking first for the kind of mouse you had. As Bensheim suggested, in your case cleaning might just do the trick.

You might find this TechSurvivors thread interesting. You'll see that I got fed up with the constant clogging of the scroll wheel (it would scroll up but not down) and Apple gave me a new mouse.

I had to clean my Mighty Mouse every month or so. Eight months sounds more like record longevity without cleaning.

I tried that site and held the mouse upside down and rolled the primary button back and forth a few times as suggested and low and behold, it is now working again.
This, after I had phoned apple and arranged to pick up a new one today. I always cover my keyboard and mouse after use so maybe that's why it lasted as long as it did.
Jon, is that rubbing alcohol you used?
Thanks,
jaybass

Yes. (It sure wasn't vodka.)

What a pity!

jaybass

a great many mighty mice (wireless and wired) have had problems with their balls specifically not scrolling down anymore.

Sometimes it's a bit of fuzz/lint down in the mechanism. You can try cleaning it, but it usually has to be replaced. If it's in warranty they will replace it for free. If not, you can try calling and complaining and maybe get it done since it's such a common issue.