A study was carried out in 49 flats in Bucharest inhabited by 265 persons. A quantitative analysis of fungi spores was performed as well as allergologic tests with Penicillium and Aspergillus fungi (A13), Aspergillus niger (M2), Alternaria tenuis (M1), Cladosporium herbarum (M3), Dermatophagoides pteronissinus (allergenic products supplied by the Bencard firm). The infested flats presented a high percentage of Aspergillus niger spores, particularly pathogenic. The main triggering factors for the appearance of the respiratory allergenic syndromes were found to be the conditions of humidity and the inadequate ventilation. Children are more affected by exposure to these conditions of the housing microclimate. Latent sensitization to fungi and to the Dermatophagoides pteronissinus mite in about 1/3 of the children and 1/5 of the adults represents a highly significant percentage of future candidates to bronchial asthma. A first requirement for the success of treatment in such subjects is the change of inhabiting conditions.