The relationship between exposure to aeroallergens and the acquisition of allergy and asthma has been shown over the past ten years. Thanks to new developments for detecting major allergens amongst the principle aeroallergens, in future it will be possible to measure their airborne concentration and to determine the particle size of particles carrying them. We report the results obtained from three studies in which we have shown that airborne mites Group I and II and cockroaches Bla g 1 and Bla g 2 allergens have a broadly similar airborne behaviour. That is to say that they are not measurable unless the atmosphere is artificially disturbed and they are carried principally on particles of > 10 microns. On the other hand, 30-40% of cat allergens are carried on particles < 5 microns and measurable in the air without any artificial disturbance. According to our data and those of the literature, we propose a different classification for aeroallergens according to characteristic aerodynamics. Although some progress in the standardisation of techniques for sampling airborne allergens may have been accomplished, other studies are required to improve their reliability in order that airborne measurements can become a marker of allergic exposure in both domestic and occupational environment.