codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>

    <script language="javascript" type="text/javascript">
// <!CDATA[

function Button1_onclick() 
{
    var cks = document.forms[0].trattamento;
    var checked = false;
    for(var i = 0; i < cks.length; i++)
    {
        if(cks[i].checked)
        {
            checked = true;
            break;
        }
    }
    alert(checked);    
    return checked; 
       
}

// ]]>
    </script>
</head>
<body>
    <form>
    <input type="checkbox" name="trattamento" value="1">
    Solo camera
    <input type="checkbox" name="trattamento" value="2">
    Camera e colazione
    <input type="checkbox" name="trattamento" value="3">
    Mezza pensione
    <input type="checkbox" name="trattamento" value="4">
    Pensione completa
    </form>
    <input id="Button1" type="button" value="button" onclick="return Button1_onclick()" />
</body>
</html>