Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/05/31/setting-the-leading-on-a-text-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            private function init():void {
                slider.value = txt.getStyle("leading");
            }
        ]]>
    </mx:Script>

    <mx:String id="lorem" source="lorem.txt" />

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="leading:" direction="horizontal">
                <mx:HSlider id="slider"
                        minimum="0"
                        maximum="10"
                        snapInterval="1"
                        tickInterval="1"
                        liveDragging="true" />
                <mx:Label text="{slider.value}" />
            </mx:FormItem>
            <mx:FormItem label="height:">
                <mx:Label text="{txt.height}" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:Text id="txt"
            text="{lorem}"
            leading="{slider.value}"
            textAlign="justify"
            width="400"
            preinitialize="init();" />

</mx:Application>

View source is enabled in the following example.

You can also set the leading style using an external .CSS file or <mx:Style /> block, as seen in the following example:

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/05/31/setting-the-leading-on-a-text-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white">

    <mx:Style>
        Text {
            leading: 12;
        }
    </mx:Style>

    <mx:Script>
        <![CDATA[
            private function init():void {
                lbl.text = txt.getStyle("leading");
            }
        ]]>
    </mx:Script>

    <mx:String id="lorem" source="lorem.txt" />

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="leading:">
                <mx:Label id="lbl" creationComplete="init();" />
            </mx:FormItem>
            <mx:FormItem label="height:">
                <mx:Label text="{txt.height}" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:Text id="txt"
            text="{lorem}"
            textAlign="justify"
            width="400" />

</mx:Application>

Or, you can set the leading style using ActionScript, as seen in the following example:

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/05/31/setting-the-leading-on-a-text-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.events.SliderEvent;

            private function init():void {
                slider.value = txt.getStyle("leading");
            }

            private function slider_change(evt:SliderEvent):void {
                txt.setStyle("leading", evt.value);
            }
        ]]>
    </mx:Script>

    <mx:String id="lorem" source="lorem.txt" />

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="leading:" direction="horizontal">
                <mx:HSlider id="slider"
                        minimum="0"
                        maximum="10"
                        snapInterval="1"
                        tickInterval="1"
                        liveDragging="true"
                        change="slider_change(event);" />
                <mx:Label text="{slider.value}" />
            </mx:FormItem>
            <mx:FormItem label="height:">
                <mx:Label text="{txt.height}" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:Text id="txt"
            text="{lorem}"
            textAlign="justify"
            width="400"
            preinitialize="init();" />

</mx:Application>