In this report we summarize our experiences based on the gene rearrangement study of 111 leukaemic patients of different kind. The lymphocyte DNA of the patients was studied for rearrangement of the immunoglobulin light chain constant-, the heavy chain joining- and the T cell receptor beta chain constant region. Our data have well supplemented the results of the monoclonal antibody experiments. In 33 cases the DNA study was in good agreement with the immunological data. In 42 our data helped in gave different results, immunological results. In 11 cases evaluating the DNA and immunological data indicating the necessity of further investigation. The results were inconclusive in 25 cases. As a conclusion we consider the gene rearrangement study to be useful for diagnostic purposes.