It is unknown whether factors such as the nature of the agent, gender, age, atopy, smoking habits, continuous or noncontinuous exposure, and pattern of asthmatic reaction can influence the rate of development of symptoms in subjects with occupational asthma. We compared several clinical and functional parameters among three groups of subjects with occupational asthma caused by Western red cedar (group 1, n = 433), isocyanates (group 2, n = 107), and high molecular weight agents acting through an IgE-mediated mechanism (group 3, n = 121). Survival analysis showed that the three curves relating years of exposure before onset of symptoms to the proportion of subjects without symptoms were significantly different in two respects: (1) almost 40% of subjects in groups 1 and 2 as compared with 20% of subjects in group 3 became symptomatic within 1 year of exposure; (2) after 5 years of exposure, the rate of sensitization was slower for subjects in groups 2 and 3 as compared with those in group 1. Having a nonimmediate reaction at the time of specific inhalation challenges, being continuously exposed and being younger slightly increased the risk at each time point on the curve of developing symptoms in subjects with occupational asthma. These data suggest that the natural history for onset of occupational asthma is different depending on the sensitizing agent. Factors such as age, type of exposure, and pattern of reaction on exposure to the agent also modulate the rate of development of this condition.