Introduction: In this study in patients with occupational asthma (OA), we aimed to determine what percentage of them have severe asthma and whether there are any variables that can predict asthma severity.Methods: We studied all patients diagnosed with OA with positive SIC between 2006 and 2013. We analysed atopic status, type of causative agent, different latency, pulmonary function tests, SIC results and inflammation in sputum. Data expressed as mean (range).Results: We analysed 83 patients (44 men, mean age 43 ± 10 years), in whom asthma was classified as "not severe" (intermittent, mild or moderate) in 62 (75%) and as severe in 21 (25%). Patients with severe asthma were older, with a mean age of 49 years (range 27-61) and had more exacerbations, with a monthly average of 0.12 (0 - 1.67); patients with non-severe asthma had a mean age of 41 years (range 22 - 65) and a monthly mean of 0.01 exacerbations (0 - 12:27) (p = 0.009 and 0.0001 respectively). FEV1 and FEV of patients with severe asthma were also lower: FEV1 78% (53-109) and FVC 81% (55-114), while non-severe asthma patients had a mean FEV1 of 92% (63-124) and a mean FVC of 92% (670 - 122) (p = 0.003 and 0.005 respectively). Finally, patients with severe asthma had a higher percentage of eosinophils 1 (0-24) in sputum than patients with non-severe asthma: 0 (0 - 55) (p = 0:04). A logistic regression showed that age at the time of diagnosis and the number of exacerbations per month were the only predictors of asthma severity (odds ratio 0.93 and 0.2, p = 0.046 and 0.004 respectively).Conclusions: One in four patients with OA may have severe asthma. In contrast to previous reports, in our study the type of agent and symptom latency do not seem to influence severity.