Hair insertion causes pilonidal sinus, it prevents spontaneous recovery, delays healing of any wound in the depth of the natal cleft, and is the cause of recurrence. An understanding of the hair insertion process made it possible to avoid hair insertion in 6545 cases of the condition with the use of the advancing flap operation. Results have proved this to be an easy and successful way of treating and preventing recurrence of pilonidal sinus. Furthermore, that understanding has introduced the possibility of preventing pilonidal sinus, through ways simpler than the simplest operation.