Seventy-two patients with haematological malignancies were treated prophylactically with itraconazole during remission induction therapy. The incidence of proven fungal infections was 18%, of which 12.5% were fatal. Aspergillus, Torulopsis and Candida proved to be major invasive pathogens. Plasma levels of itraconazole were monitored for all patients. The occurrence of fungal infection is significantly greater in the group where no therapeutic plasma levels were obtained during at least two weeks. There is a clear need to obtain quick information on itraconazole plasma levels in order to adapt dosage during prophylactic treatment in immunocompromised patients. The influence of itraconazole on liver function tests could not be separated from concomitant cytostatic or antibiotic treatment. No jaundice directly related to itraconazole could be found. During itraconazole prophylaxis a shift from classic pathogens such as Aspergillus and Candida, to Fusarium, Torulopsis and Mucor, may be seen.