Ugly Lines

Hi!
I am displaying GL_QUADS as
glLineWidth(1);
glEnable(GL_LINE_SMOOTH);
glPolygonMode(GL_FRONT_AND_BACK,GL_LINE);
But sometimes the lines do not look very pretty. Some times the lines are broken.
I do not know??
Juergen

Did you enable blending?

Try also glHint(GL_LINE_SMOOTH_HINT, GL_NICEST) to ask for best quality (this is only a hint, it can be ignored by the implementation).