Companies are wanting to know if (a). you made more then than you will now - being underpaid; or (b). you made less then than you will now - being overpaid. If your previous salary was way above or way below what they were looking at for the position, it can be a negative.

If you were being paid more, then they will think you will be unhappy with a lower salary and are unlikely to hang about. If you were being paid less, they will think that you might not be worth the extra money - since the previous market rate was lower.

Of course, if you don't tell them what you were making before, there will be a certain level of distrust embedded in their minds.