Hi there,

It depends what you mean by "under your knee". The commonest is a lump on the front of the leg, where the tendons (patella tendon) goes over the front of the knee and inserts into the tibia - the shin bone.

If you are active when young, the point where the tendon goes in at the top of the tibia (shin bone) can get larger - if severe often called Osgood-Schlatter disease.

If it is a problem, you need to talk to a knee specialist - usually an orthopaedic surgeon or sports specialist - look-up the Specialist Websites page of Medical Pages and click on the Orthopaedics link.

Mark