Cases can be dismissed for insufficient evidence without first suffering a conviction. That would not automatically mean "not guilty", although I suppose that is entirely possible. If there is an arrest, conviction and then a dismissal it probably means the court agreed to expunge the conviction.
A dismissal can happen for various reasons. However a dismissal is NOT a conviction and it is not a not guilty verdict.
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Dismissal does not mean there was a conviction and it does not mean not guilty.
Of course, every DUI case is different and you should consult an experienced DUI defense attorney in your area with questions regarding your specific case. It is always in your best interest to have a good DUI attorney represent you when you have been arrested for a DUI.