You don't really need to get xmlHttp every time an AJAX call is made. That can be done outside the function upon page load. I also think you should have done onreadystatechange to a function that handles the response, like this:

...
element = 'mydiv';
xmlHttp.onreadystatechange = handleResponse;
xmlHttp.send(null);

function handleResponse()
{
    if(xmlHttp.readyState == 4)
    {
        var response = xmlHttp.responseText;
        document.getElementById(element).innerHTML = response;
    }
}

This example will only work if xmlHttp and 'element' are declared globally.