Cells of a 21-year-old patient with acute lymphatic leukemia were analyzed for morphology and immunophenotype and for genotype consecutively during the course of disease. Initial therapy with the BMFT-ALL protocol (Bundesministerium für Forschung und Technologie) reduced leukemic cells only marginally. The following high-dose Ara-C, mitoxantrone (HAM) chemotherapy led to a cell reduction of 75% and to a drastic change in cell morphology from initially 90% blasts to mainly small lymphoid cells. Immunophenotype, which showed 90% CD7-positive cells in the beginning with a prevalence of helper (60%) over suppressor cells (15%) remained fairly constant until the onset of HAM chemotherapy, which led to a sharp fall and a subsequent slow increase in all T-cell markers. In contrast to pretherapeutic findings, CD7 was now only expressed on the small cells and not on blast cells. Southern blot analysis of the T-cell receptor configuration revealed an initially monoclonal population with rearranged T beta gene. A new band appearing during the clinically ineffective therapy was indicative for development of a second small population which did, however, not emerge in immunophenotype analysis. This second population was eliminated by the HAM chemotherapy, leaving back the initial clone responsible for the final fatal outcome. No activity of the multidrug resistance gene could be detected by Northern blotting.