A total of 431 workers from 14 leather factories located in the Tuzla Organized Industry Region, Istanbul were involved in the study. Subjects were interviewed and examined individually with regard to asthma symptoms. Physical examinations were undertaken and respiratory function measured by spirometer. Moreover, the atmosphere of the working areas in the factories was assessed microbiologically. The fungal genera most often detected were Penicillium spp., followed by Aspergillus spp., Alternaria spp., Scopulariopsis spp., and Cladosporium spp. multiplying at differing rates. The number of those physically examined with indications of peripheral respiratory obstruction was 176 (40.8%) while those with suspected asthma following questioning was 153 (35.5%). The obstruction demonstrated by the use of respiratory function tests was associated with the mold isolated from the environment and was included in the logistic model that was developed as a risk factor for increasing obstruction. Workers employed in tanneries encounter many health problems. These may be reduced by: improving the factory environment, environmental control of allergens, elimination of allergen reservoirs, control of humidity and reducing exposure to excessive heat or cold. In addition there should be air filtration and vacuum cleaning to control dust mites and control of other workplace risks not necessarily associated with tanning. Importantly, workers should have medical examinations before employment and periodic examinations during training and employment to reduce the risk of occupational diseases to a minimum.