I. INTRODUCTION The popular characterization of completing and mailing individual tax forms is that people wait until the last minute to fill out their returns, and then rush to mail them at their nearest post office, which has extended its hours until midnight. This phenomenon, which we call "April 15 syndrome," has large private costs. We estimate that for tax year 1988 the 73.2 million taxpayers claiming refunds gave up nearly one billion dollars in interest, or an average of about $13.50 per return, due to filing the forms later than the earliest possible time.(1) More incomprehensible from an economic standpoint, but of less quantitative significance, are the impatient tax filers - 10.5 million taxpayers who owed the Internal Revenue Service (IRS) money and passed up $46 million in interest by mailing in their taxes before the filing deadline. In addition to these direct costs, tax returns done in haste at the last minute may be more prone to error; to the extent they are not caught by the IRS, this adds to the capriciousness of the tax burden. Correcting the errors due to rushed completing of forms also adds to the administrative costs of the IRS. Many cities and towns keep their post offices open late for no reason other than to accommodate the severe procrastinator. Finally, return processing is slowed by the avalanche of returns filed in mid-April. We present some exploratory analyses of April 15 syndrome using 1988 tax return data. After describing in section II the legal framework for tax filing, in section III we discuss the data and present the basic facts with which any model of filing behavior has to contend. There is a wide distribution of filing times, and a substantial fraction of households which, despite being owed refunds, file late, or which, despite owing taxes, file early. In section IV, we consider some basic models of the decision as to when to file one's taxes. We note that the simplest model fails to rationalize the most obvious characteristic of the data the substantial heterogeneity of filing times. In the baseline rational model individuals simply maximize their income less effort, which involves either complete procrastination or prompt filing. Adding imperfect information does not seem to eliminate the result. We are led to a model in which individuals have a stochastic opportunity cost of doing their taxes. This model generates a wide distribution of filing times for returns which will get refunds and rationalizes procrastination. However, our framework fails to generate early filing of returns with taxes due, and we consider the issue again after our analysis of the data. In section V, we take the predictions of our model to the data. We examine the various characteristics of refund and remittance returns and perform some reduced-form regressions of the determinants of return filing time. Most supportive of our interpretation of late filing for refunds, there is much less procrastination for returns which are filled out by a paid preparer, than for returns which are filed by the taxpayer. Further, we find that the complexity of forms delays filing, both for returns with tax due and for those with refunds. People with a higher marginal valuation of time, proxied by higher incomes, are more likely to file later, and the larger the refund value, the sooner the return is expected to be filed. Returns which are completed by professional tax preparers do not exhibit these last two characteristics. Finally, cross-year analysis demonstrates that filing late is habitual, suggesting that households have persistently different values of time or propensities to procrastinate. We conclude by speculating on the objectives and constraints which might generate early filing by individuals with taxes due and by considering the implications of our findings. We believe that our investigation of the timing of tax return filing informs us about other examples of apparently non-optimal timing of economic behavior. …