Jay Leno scored big ratings Monday night, his first back as host of The Tonight Show.

Leno led the 11:35 p.m. Eastern time slot with 6.6 million viewers, beating both ABC's Nightline, which had 4.1 million viewers, and CBS' Late Show with David Letterman, which had 3.8 million viewers, according to ratings information provided to NBC by Nielsen.

The 6.6...