Display WordPress Parent Categories Only?

This should do what you’re looking for. It’s untested, I just whipped it up really quick right now, but the concept is there.

function my_get_highest_parent( $id ) {
    $cat = get_category( $id );
    $parent = $cat->parent;

    if( $parent == 0 ) 
        return $id;
    else
        my_get_highest_parent( $parent );
}