When must the patent owner pay the infringer?

Large sums of money -- sometimes even tens or hundreds of millions of dollars - have been collected by patent owners who have sued for patent infringement. Lest patent owners become overconfident, however, suing for patent infringement can sometimes have the opposite effect: the patent owner can find itself liable to the accused infringer for a large sum. A recent lawsuit shows how this can happen - the patent owner sued an infringer and lost, and the infringer responded with a counterclaim against the patent owner under the antitrust laws. The counterclaim succeeded, resulting in a judgment of $10 million in favor of the infringer.

The patent (U.S. Patent No. 4,330,891) was for …