eg.

<custom:DynamicFormItem id="lblIdentIdType" label="Identification Type:" required="true" visible="{grpUSCitizen.selection == rdoCitizenNo}">
<mx:ComboBox id="cboIdentType" dataProvider="{identType}" toolTip="Required Field" />
<mx:Spacer height="0"/>

package custom
{
import mx.containers.FormItem;

public class DynamicFormItem extends FormItem {

/**

• Handles dynamic height of this component.
  */
  public override function set visible(visible:Boolean):void { super.visible = visible; if(!visible) height = 0; else height = undefined; }
  }
  }