It is important that you consult with an attorney. Even if an arrest did not result in a conviction, it still may have possible immigration consequences. The same applies to prior convictions which were later expunged or removed from your record by the court due to passage of time, etc. In order for an attorney to fully assess what impact such an event will have on your ability to immigrate to the U.S., you will need to secure original or court certified documents which indicate the charges filed, the sentence imposed and the final disposition of the case. These documents will also need to be provided as part of your application for permanent residency.