<select name="id_klwkjw" onchange="dodaj_srednice_mpm3()" id="id_klwkjw" validate="required:true">
option
..

</select>

function dodaj_srednice_mpm3()
{
    var ajaxRequest;
    try
    {
        ajaxRequest = new XMLHttpRequest();
    }
    catch (e)
    {
        try
        {
            ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
        }
        catch (e)
        {
            try
            {
                ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
            }
            catch (e)
            {
                alert("Twoja przegladarka nie obsluguje javascript!");
                return false;
            }
        }
    }
    ajaxRequest.onreadystatechange = function()
    {
        if(ajaxRequest.readyState == 4)
        {
            var ajaxDisplay = document.getElementById('srednica_mpm3');
            ajaxDisplay.innerHTML = ajaxRequest.responseText;
        }
    }
    var id_klwkjw = document.getElementById('id_klwkjw').value;
    var queryString = "?id_klwkjw=" + id_klwkjw;
    ajaxRequest.open("GET", "wykaz_srednica.php" + queryString, true);
    ajaxRequest.send(null);
}