A pilonidal sinus is a small hole or channel in the skin that usually occurs at the top of the cleft between your buttocks.

It may not cause any noticeable symptoms unless it becomes infected.

Pilonidal sinuses have also been known to develop on the belly button or between fingers. In many cases, there is more than one channel.

If the sinus becomes infected, an abscess will usually develop. An abscess is a painful collection of pus that causes symptoms such as:

• pain
• a tender lump under the skin
• redness of the skin
• pus draining from the sinus – the pus usually smells unpleasant
• blood draining from the sinus 
• a high temperature of 38C (100.4F) or above, although this is uncommon

These symptoms may develop quickly, often over a few days.

The pain caused by a pilonidal sinus can be severe and is likely to get worse without treatment. This can make it difficult to sit or lie down comfortably, which may affect your sleep. The pain may also affect your ability to work and your daily activities.

See your GP as soon as possible if you suspect you have the condition. They can examine you and discuss possible treatments.

Read more about treating pilonidal sinus.