Recently, we and others have shown the occurrence of TCR delta gene rearrangements in acute myeloid leukaemia (AML). In this study we describe the molecular characteristics of these rearrangements by the polymerase chain reaction (PCR) and the direct sequencing of PCR products. 11 rearrangements were characterized in blast cell samples from six patients. We found a heterogenous pattern of TCR delta gene rearrangements with involvement of V delta 1-5 regions. These findings differ from observations in T-ALL and B-cell precursor ALL, where predominantly usage of V delta 1 and V delta 2 regions has been described. Furthermore, extensive diversity of junctional sites was observed, including addition of up to 37 N nucleotides, nucleotide deletions at junction sites of V delta and J delta segments and usage of up to three D delta segments. The D delta 3 fragment was the most frequently used diversity element and was found in 10 rearrangements. Nine of the 11 rearrangements were non-functional, either incomplete or out of the reading frame. Therefore a functional TCR delta cannot be expressed in these myeloid blast cells.