This contribution concerns the issue of the impact of the adjustment for outstanding claims on the method of registration of business revenues and costs. The subject of the analysis undertaken in this study is therefore the adjustment for outstanding claims regulated by the Personal Income Tax Act, Corporate Income Tax Act and a Flat-rate Income Tax on Certain Revenues Earned by Natural Persons Act. The purpose of this study is to show that the adjustment for outstanding claims changes classification of revenues and costs from business activities. An objective research difficulty is to indicate the moment when the right or obligation arises. Therefore, the coherence and completeness of regulations governing the adjustment for outstanding claims in income taxes was examined.