The most frequent mistake: more than 900,000 failures to subtract last year's advance payment from the credit amount. Those who cannot recall how much their advance payment check was can easily check the IRS Web site for the amount. Another 450,000 returns reflected incorrect calculations of this tax credit.

Taxpayers can find their advance payment amount on IRS Notice 1319, which they should have received just before the checks. Those who do not have this notice can easily get the amount through IRS.gov or by calling the toll-free IRS help line, 1-800-829-1040.

Last year's payments were an advance on the credit's increase for 2003, when the maximum per-child amount rose from $600 to $1,000.

Taxpayers must put the amount of their advance payment check on line 2 of their Child Tax Credit Worksheet. Taxpayers use this worksheet, found in the instructions for Forms 1040 and 1040A or in IRS Publication 972, to figure the amount of credit they can claim on their 2003 tax returns.

Taxpayers claiming the Child Tax Credit who did not receive an advance payment will get their entire benefit from the credit on their returns. Taxpayers whose advance payment was larger than the amount of their credit will not have to repay the difference and will not claim any Child Tax Credit on their 2003 returns.

If the advance payment was reduced because of past-due taxes or certain non-tax debts, the taxpayer must use the full advance amount -- before offset -- in the worksheet. This is because the person received the benefit of the advance payment amount that was used to pay the past-due amount.

Separately, the IRS said e-filing continues to show a strong increase, with e-filed tax returns running more than 10 percent ahead of last year. The biggest increase is being seen in home computer use, which is up 23 percent.