The clinical effects and tolerability of micafungin sodium in daily practice for the treatment of fungal infection in critically ill patients were evaluated in an open-labeled, non-comparative, observational study. All patients admitted to intensive care units (ICUs) of 3 hospitals in Chiba prefecture between June 2003 and March 2005, who were treated with micafungin because of known or suspected fungal infection, were included in the study. A total of 34 patients received micafungin and 29 cases of them were subjected to analysis. Fungal infections were classified as "proven" in 3 patients (10.3%) and "possible" in 26 (89.7%). Candida was detected in 16 patients, most of them were Candida albicans and 4 cases were non-albicans Candida. Clinical effects of micafungin were "cured" and "improved" in 20 patients (77%), "failure" in 6 (23%), and "undetermined" in 3 cases. Adverse events were reported in 10 patients, but there was no significant event. In conclusion, micafungin was effective in 77% of proven or suspected fungal infections in critically ill patients admitted to the ICU. The drug was well tolerated and discontinuation of its treatment due to adverse events was not experienced during the study period.