This study was planned to determine the effect of building constructions in and around our hospital, on the development of opportunistic mold infections in immunocompromised patients hospitalized in bone marrow and kidney transplantation units and haematology and oncology units. Samples were collected from high risk units by an air sampler (Air Ideal) from indoors and outdoors of a total of 43 patient rooms. The most commonly isolated species from indoor air cultures of our hospital were Penicillium spp. (50.6%), Cladosporium spp. (20%), Chrysonilia spp. (11%) and Aspergillus (10.6%) species. When outdoor samples were considered, Penicillium spp. (38.8%) was still in the first line, followed by Cladosporium spp. (24.3%), Paecillomyces spp. (10.7%) and Aspergillus (8.7%) species. There was no statistically significant difference of total colony and spore numbers between the samples obtained from indoor and outdoor air (p > 0.05), indicating the close relation with the construction studies in and around the hospital. Clinical samples including bronchoalveoler lavage (BAL) fluid, sputum, endotracheal aspirate and sinus tissue were collected from the total of 43 patients staying at these air sampled rooms, and eight of them (18.6%) yielded positivity for the growth of molds. Of them four were identified as Penicillium chrysogenum (sputum isolates), two as Aspergillus fumigatus (sputum and BAL isolates), one as Aspergillus flavus (BAL isolate), and one as Valsa sordida (sinus tissue) which is considered as a plant pathogen. A total of 53 sera, BAL, and tissue supernatant samples were screened by ELISA for the presence of galactomannan antigen, and five of the eight patients whose cultures were positive were also found positive for galactomannan antigen. One patient has died due to invasive aspergillosis whose BAL specimen and indoor air sample were positive for A. fumigatus. In evaluation of indoor air samples before and after the change of HEPA filters, statistically significant decrease was detected in total colony and spore numbers between the samples taken before and after the filter changes (p < 0.005). This study has emphasized the importance of examination of mold flora of indoor air and clinical samples of high risk group patients intermittantly, in order to prevent opportunistic mold infections in crucial units especially during hospital constructions.