We investigated the significance and the usefulness of monitoring plasma voriconazole levels in patients with chronic necrotizing pulmonary aspergillosis associated with underlying chronic respiratory diseases. The average trough level was 2.2 microg/ml and there was no correlation between trough levels and voriconazole doses. Orally administered drug showed no significant difference in trough or peak levels compared with parenteral injection. Six cases with visual adverse events had significantly higher nadirs compared to those without visual disturbance. All three cases who discontinued the drug due to liver dysfunction had plasma trough levels higher than 4.0 microg/ml. Those who failed to respond to the treatment had trough levels lower than 1.4 microg/ml or peak levels lower than 2.8 microg/ml, while some cases with plasma level lower than those levels responded well. Since plasma voriconazole level has a large inter-patient variability, drug monitoring may be beneficial to evaluate the drug efficacy and safety in each individual.