What is the top disability amongst US military personnel?

Noise-induced hearing loss is the top disability amongst US military personnel.

Answer Extra: 

According to the Deafness Research Foundation, more than 65% of returning combat troops from Afghanistan suffer from noise-induced hearing loss. Many troops sustain acoustic trauma – hearing loss – when exposed to very loud blasts, such as incendiary explosive devices (IEDs).

source: 
Deafness Research Foundation. “Hearing loss number one diagnosis for military.’ http://www.drf.org/news/161/Hearing+loss+number+one+diagnosis+for+military