When there's an invisible man sleeping in your hospital bed, who ya gonna call?

Oh, come on. YOU KNOW WHO.

Kristen Wiig, Leslie Jones, Kate McKinnon and Melissa McCarthy -- who are currently filming "Ghostbusters" in Boston -- made a surprise appearance at Tufts Medical Center in full costume (including their proton packs, just in case any wayward spirits happened to be visiting at the same time.)

Although the Ghostbusters do have a secretary monitoring their call line, in this case they were summoned directly by some creative staff and patients, who posted flyers in the hospital's windows that read, "WHO YA GONNA CALL?"

Meanwhile, the appearance of the Ghostbusters at Tufts was just the latest example of Hollywood's finest taking time out of their schedules to do a hospital cameo (although it might have been the first time a celebrity team checked under the beds for ghosts.)

Below, we've rounded up some of the other heartwarming times that celebs have showed up for sick kids in the past year or so.

When Jennifer Lawrence took a Christmas trip to the ICU.

The pediatric intensive care unit at a local Louisville hospital got a special Christmas Eve visit from the "Hunger Games" star.

When Johnny Depp was the best, worst pirate you've ever heard of.

Depp went full-on Captain Jack Sparrow for this trip to Lady Cilento Children's Hospital in Australia, where he stayed to record a segment for a show produced by the kids on the ward.

When Taylor Swift put on a concert for one.

A six year-old fan with leukemia was the sole audience member for TaySway's special, exclusive performance at Boston Children's Hospital.

When Selena Gomez joined a special team.

The star visited a Los Angeles children's hospital and even added a new accessory to her collection in support of one of the patients she met.

When Chris Pratt stole his Starlord costume just for hospital visits.

Spoiler alert: You're gonna see a lot of @prattprattpratt on this list.

When Chris Evans spent the holidays with one special fan.

The actor left his Captain America costume at home, but took time out of his busy schedule to hang out with a 9 year-old fan at Christopher's Haven in Boston.

When Chris Pratt lost a bet, and won our hearts.

And speaking of Chris Evans: His friendly Superbowl rivalry with Chris Pratt resulted in Pratt showing up to Christopher's Haven himself, in full "Guardians of the Galaxy" regalia.

When Chris Evans won the same bet, and also won our hearts.

The first Avenger happily suited up for a trip to Seattle Children's Hospital, even though he technically didn't have to.

And when Chris Pratt showed up for no particular reason except that he cares, and he's awesome.

Most recently, the actor used his velociraptor-wrangling skills on a few wily patients during a visit to Our Lady of the Lake Children's Hospital in Louisiana.