This report presents the results of a randomized parallel design comparative study of the serum concentrations of fluconazole and itraconazole after administration of 100 mg orally to patients with leukaemia. Each group consisted of ten patients. The antimycotic drugs were administered with a standard breakfast immediately before the start of chemotherapy (day one) and on days eight and fifteen. No significant differences (p > 0.05; ANOVA) in the pharmacokinetic parameters of fluconazole (AUC, Cmax, Tmax) were found during the three days of the trial. It is concluded that there is no clinically important pharmacokinetic interaction between fluconazole and the chemotherapeutic agents given to this group of patients. A pharmacokinetic interaction between fluconazole and the fever suffered by some of the patients also seems unlikely. No significant differences (p < 0.05; ANOVA) in the pharmacokinetic parameters of itraconazole (AUC, Cmax, Tmax) were found during the three days of the trial, although the statistical power of the data was low. The significantly greater variability of all pharmacokinetic parameters for itraconazole than for fluconazole and the sharp increases and decreases in AUC during the course of the trial found for some patients in the itraconazole group suggest the need for caution in this group of patients.