General reviews about occupational asthma divide the susceptible allergens capable of initiating asthma into two distinct groups: those of high and of low molecular weight. Atopy would be a risk factor for developing occupational asthma to high molecular weight allergens but not to those of low molecular weight. In this work we have examined several studies and have analysed in a critical manner the relationship which exists between atopy and occupational asthma. The high molecular weight allergens studied were: snow crabs, laboratory animals, flour, proteolytic enzymes and psyllium. Those of low molecular weight were: red cedar, isocyanates, phthalic anhydride. Amongst allergens of high molecular weight there is a undeniable relationship between atopy and sensitisation to the allergen incriminated. However, the relation between atopy and asthma is more debatable except for baker's asthma. For more allergens of low molecular weight, atopy does not seem to favour the appearance of asthma, however, it may play a role in the occurrence of asthma to red cedar. The difference between the two groups of allergens is not as clear-cut as in the work that has appeared up until now and the exclusion of atopics from being hired would appear excessive.