Yes. You can change it anytime. You can even download the form off the IRS.gov website, print it, fill it out, and submit it to your HR Dept or person. If you had to pay additional taxes with your tax return for '10, and prefer not to next year, claim less allowances, which will mean your employer will withhold & pay more taxes on your behalf from each paycheck through the year. Let me know if you need any more info or advice on how many exemptions/allowances to put on the form.