In order to study the occurrence of t(12;21)-positive cases in childhood ALL in the Czech Republic, we established a reverse transcriptase polymerase chain reaction (RT-PCR)-based method for the detection of hybrid TEL/AML1 mRNA. We studied bone marrow (BM) and/or peripheral blood (PB)samples from 167 unselected patients with ALL younger than18 years, newly diagnosed between March 1995 and January 1998.