The States that allow Monkeys to be kept as pets legally are Washington state, Montana, Nevada, North Dakota, Nebraska, Kansas, Iowa, Missouri, Arkansas, Wisconsin, Illinois, Ohio, Alabama, West Virginia, Virginia, North Carolina and South Carolina.
Those are all the States that it's legal to keep a Pet Monkey.
The rest of the States it's illegal to have Pet Monkeys as far I know.
If you live in any other state than the states listed above then check with your local laws to be sure of the legality of keeping a pet monkey.