Captain Steve Edmondson said about 8% of the 585 detainees at Camp Delta were suffering from mental disorders, with half of that figure requiring medication.
He said about 15% of the inmates arrived at the prison with mental disorders of some kind and about 8% were being treated for psychiatric disorders now.
He added that inmates suffered from schizophrenia, depression and personality disorders "of the kind that you would see in an any adult population".
Edmondson was questioned about allegations by former inmates that Australian detainee David Hicks, whose military tribunal is to start this week, was abused while at the camp on a US naval base in Cuba.
The Australian government has asked US authorities to investigate the allegations but no details of the inquiry have been given. The US military has denied that there has been abuse at the camp.
The doctor said "we haven't had investigators come down to take statements. We have been asked to provide medical summaries in some cases", in the form of written statements.
Most of the inmates were detained in Afghanistan in late 2001
and have been at Guantanamo since January 2002.
The authorities have reported some 34 suicide attempts by 21 detainees in that time.
There are five permanent patients in the camp hospital, one
recovering from a suicide attempt and the others from surgery.
Rights groups have criticised US authorities for keeping the
detainees in virtual secrecy and Edmondson acknowledged "there is some anxiety about what the future might hold".