The aim of this study was to investigate the progression and long-term consequences of occupational asthma and to evaluate the effect of the pattern of response to specific bronchoprovocation test (SBPT) in the prognosis of occupational asthma among 40 subjects with asthma due to toluene diisocyanate (TDI). Nine subjects had immediate asthma, 25 subjects had late asthma and 6 subjects had dual asthma. Respiratory symptoms, persistence of exposure to TDI and lung function have been investigated both at diagnosis and at follow-up examination. At follow-up examination, 70.0% of the subjects were not exposed to TDI, and only 14.3% of "non-exposed" subjects were completely asymptomatic. At follow-up examination, 71.4% of "exposed" subjects and 55.6% of "non-exposed" subjects were regularly treated with medications because of their asthma (no statistical difference). No significant differences have been found in symptoms prevalence or lung function between "exposed" and "non-exposed" subjects. After comparing the results obtained at diagnosis and at follow-up examination, a significant decrease in FVC both in "exposed" and "non-exposed" groups (p < 0.005 and p < 0.0005, respectively) and a significant decrement in FEV1 (p < 0.05) and FVC (p < 0.0005) among the subjects with late asthma, was found. We can conclude that, despite the removal from TDI exposure, occupational asthma can lead to permanent disability with important socio-economic consequences, and late response to SBPT may be a negative prognostic factor in TDI asthma.