The mite fauna in hay dust of 16 farms located on the island of Gotland was analysed to establish whether there is a dear relationship between environmental exposure to storage mites and development of allergic symptoms. Eight of these farms belonged to farmers with respiratory symptoms related to barn dust (RS) and eight to healthy farmers (H). All farmers had previously participated in an epidemiological survey of hypersensitivity to respiratory allergens. The symptomatic farmers had not only a positive RAST to storage mites but they developed more than a 20% fall in FEV, when challenged with Lepidoglyphus destructor (L. destructor) extract. The asymptomatic farmers were all negative in RAST to storage mites. Mites were found on all farms and the degree of infestation varied enormously, mite counts ranging from 75–150,000 mites/g dust. Five genera were detected but the different mite species were not represented on all farms. L. destructor was the dominating species on 13 of 16 farms. Samples from RS FARMS contained a significantly higher number of mites than H farms. The results of this study are in good agreement with our earlier studies showing that L. destructor is the most important allergen causing symptoms from both upper and lower airways among farmers.