Gene transfer of the cytidine deaminase (CDD) cDNA has recently been shown to induce cellular resistance to cytarabine (AraC) in vitro. To investigate the role for CDD in acute myeloid leukaemia (AML) we analysed the CDD activity and CDD gene structure in blast material from well-defined patients with untreated and AraC refractory (RF) AML. Median CDD activity in previously untreated AML was significantly lower than in RF-AML blasts (P = 0.015) and was significantly lower in patients with complete remission than with blast persistence following induction chemotherapy (P = 0.043). Structural investigation of the CDD gene by Southern analyses and RT-PCR showed no detectable aberrations. Sequence analysis of the CDD cDNA from nine RF-AML patients showed inconsistent aberrations in three patients. Semiquantitative assessment of CDD mRNA expression revealed a significant correlation with CDD activity. In conclusion, concordant with another recent study our data suggest a correlation of pretherapeutic CDD activity with induction treatment response. Besides the previously described prognostic impact of mdr1 expression, this result could be useful for the development of risk-adapted AML treatment strategies and warrants further studies of CDD activity in well-defined cohorts of AML patients and of the mechanisms involved in the regulation of CDD activity.