<script type="text/javascript"> checkobj = 0;</p> <p>function checkAgree(obj) { if (obj.checked) checkobj++; else checkobj--; if (checkobj <= 0) document.check_agree_form.elements['submit_button'].disabled = true; else document.check_agree_form.elements['submit_button'].disabled = false; } </script></p> <p><form action="#" method="post" name="check_agree_form"> <p> <input name="agree" type="checkbox" value="agree" onclick="checkAgree(this)"/> <b>Соглашаюсь</b><br/> <input disabled="" name="submit_button" type="submit" value="Отправить"/> </p> </form>

Демо