Aspergillus galactomannan immunoassay is a main diagnostic and monitoring tool in medical mycology. However, the specificity of the method can be skewered by the presence of several other fungi. Trying to diagnose a possible fungal infection of the lower respiratory tract in a haematology patient, it appeared that the fungus Trichoderma longibrachiatum is an additional probable cause of positive galactomannan results. Although, that Trichoderma is a rare but emerging pathogen in immunocompromised patients, the above information could be a caution point in the clinical evaluation of diagnostic results.