Our aim was to estimate the duration of maximum tolerated dose (MTD) duration for gemcitabine given as a continuous infusion in combination with fludarabine and mitoxantrone and to evaluate potential pharmacokinetic (PK) interactions in 17 patients with refractory or relapsed acute myeloid leukaemia (AML). Gemcitabine was administered at 10 mg/m(2)/min for 3-15 h, fludarabine at 25 mg/m(2) daily for days 1-5 and mitoxantrone at 10 mg/m(2) daily on days 1-3. PK studies revealed that fludarabine clearance was not affected by gemcitabine but mean terminal half-life and volume of distribution of fludarabine were slightly increased. The duration of MTD for gemcitabine was 12 h. Our previous in vitro work has demonstrated the binary combination of gemcitabine + fludarabine is most synergistic at a molar ratio around 0.002. However, with MTD dosing this drug ratio is not optimal to produce synergy and future studies using ratiometric dosing are required to confirm these findings.