In August 1988, Megadeth was at the Monsters of Rock festival in the United Kingdom. They played music for over 100,000 people there. For one show, Metallica drummer Lars Ulrich played with the band. Megadeth went on the Monsters of Rock European concert tour. They stopped playing after the first concert because of Ellefson's drug addiction. Ellefson was treated for his issues, and Testament replaced Megadeth for the rest of the tour.