function listAllStates()
{ 
  var xmlDoc=xmlHttp.responseXML;
  var allStates=xmlDoc.getElementsByTagName("state");
  outputList("All States in Document",allStates);
}
function outputList(title,states)
{ 
  var out=title;
  var currentState=null;
  for(var i=0;i<states.length;i++)
  {
    currentState=states[i];
    out=out+"\n-"+currentState.childNodes[0].nodeValue;
  }alert(out);
}

function listAllStates()
{ 

var xmlDoc;
var xmlString = '<states><state>wei</state></states>';

    // Mozilla and Netscape browsers
    if (document.implementation.createDocument) {
        var parser = new DOMParser()
        xmlDoc = parser.parseFromString(xmlString, "application/xml")
    // MSIE
    } else if (window.ActiveXObject) {
        xmlDoc = new ActiveXObject("Microsoft.XMLDOM")
        xmlDoc.async="false"
        xmlDoc.loadXML(xmlString)
    }
   

  var x = xmlDoc.getElementsByTagName('state');

  alert (xmlDoc);
  alert (x.length);
}
 
 listAllStates();

------解决方案--------------------
你保证你的远程XML读取正确么?
还有是否可以考虑使用XPATH来解决
也就是XDOC.selectNodes("//state").length;

------解决方案--------------------
JScript code
function handleStateChange()
{
  if(xmlHttp.readyState==4){
//如果你直接拖进浏览器的话正确执行时返回的状态为0,要想返回200状态需要服务器支持
//所以要加上xmlHttp.status==0的判断
        if(xmlHttp.status==200||xmlHttp.status==0){
              if(requestType=="north"){
                 listNothStates();
              }
              else if(requestType=="all"){
                 listAllStates();
              }
         }
   }
}