4 – Day Long Surgery

The longest surgery to date performed in 1951. It was a 96-hour marathon to remove a 300-lb ovarian cyst from 600-lb women. Simply hacking it out would have killed her, so the surgeon gradually drained fluid from the cyst over the course of 4 days, shrinking it to 100-lb and allowing for successful removal. 

Source: http://canyouactually.com/interesting-facts-you-never-knew/