An invasive mycosis may cause death in high-risk patients. An early systemic antimycotic therapy can save life. Therefore, a continuous mycological monitoring in one week intervals is necessary in high-risk patients beginning with the day of admission. This monitoring should be done three to five times a week when an organ manifestation is suspected. Due to the continuous monitoring, the assignment of the results is much easier for the clinician. The goal of the mycological monitoring is to obtain an early hint of a fungus infection. The results of the culture as well as serum titers of antigen and antibodies have to be interpreted in connection with the clinical picture of the underlying disease and the actual risk of infection. Negative results do not rule out a mycosis! Positive results do not always proof an invasive mycosis. Only by interpreting the time course of the mycological findings and the patient's clinical status, an invasive mycosis may be diagnosed with some certainty. In any case, additional procedures like radiological techniques (i.e. CT-scan), histology etc. should be used.