A total of 28 children and nine adults with relapsed T-ALL were analyzed for the configuration of their T-cell receptor (TCR) and TAL1 genes at diagnosis and relapse to evaluate their stability throughout the disease course. A total of 150 clonal TCR and TAL1 gene rearrangements were identified in the 37 patients at diagnosis. In 65% of cases all rearrangements and in 27% of cases most rearrangements found at diagnosis were preserved at relapse. Two children with unusually late T-ALL recurrences displayed completely different TCR gene rearrangement sequences between diagnosis and relapse. This indicates that a proportion of very late T-ALL recurrences might represent second T-ALL. Specifically, 88% of clonal rearrangements identified at diagnosis in truly relapsed T-ALL were preserved at relapse. This is significantly higher as compared to previously studied precursor-B-ALL ( approximately 70%). Thus, from biological point of view, immunogenotype of T-ALL is more stable as compared with precursor-B-ALL. The overall stability of TCR gene rearrangements was higher in adult T-ALL (97%) than in childhood T-ALL (86%). Based on the stability of TCR gene rearrangements, we propose a strategy for PCR target selection (TCRD+TAL1 --> TCRB --> TCRG), which probably allows reliable minimal residual disease detection in all T-ALL patients.