Why a late-night trip to the dermatologist? And what are they hiding?

Model Kendall Jenner who claims she's eschewed all plastic surgery and injections, hid her face as she went to the "dermatologist" at 8pm Friday night! Kourtney Kardashian didn't want to be seen either -- she wore a shocking t-shirt reading "start f-ing" and slid her way into the medical building elevator for the midnight makeover.

The chic Beverly Hills office stayed open after-hours for the private 2-hour treatment session. Of course any doctor will stay open late for the Kardashians ... with all the business they get from the family!