Any lump that appears on the body with no explanation is an unwelcome intruder. Plus, not knowing what to do if one takes shape under your arm can be irritating physically and mentally. Although the only sure way to diagnose a lump under your arm is to see your doctor and have it tested, there are some basic symptoms to look for that will help you determine whether the lump in question is a cyst.
Cysts are usually skin-colored, perhaps with a yellowish tint, and unless they are infected, they should be painless. They tend to be small growths, not getting much larger than 2 inches (about 5 centimeters) across. Some cysts exhibit either a blackhead or evidence of a skin pore that if squeezed or accidentally ruptured, will leak a cheese-textured and foul-smelling pus [source: Mayo Clinic].
Another way to check the cyst is to do the wiggle test. Take your thumb and forefinger, gently squeeze the lump and then move it back and forth. If it wiggles, that is an indication that it's probably a cyst because they tend to be easily movable [source: University of Rochester].
If you've recently experienced some sort of skin injury at or near the site of the cyst, that is also a good indicator that you are dealing with a benign growth. Acne, shaving, and even antiperspirants (not deodorant) can all cause the irritation and skin damage that leads to the formation of a cyst [source: MedlinePlus: Armpit].
If the cyst is not causing any pain or discomfort physically or psychologically, it can be left alone. However, if you are worried it is more than a cyst, or if the cyst ruptures or becomes infected, inflamed or painful, then treatment options -- described in the next section -- are available.