Our study included 27 polyurethane foam workers exposed to MDI only at low concentrations (ranging from 0.0005 to 0.001 ppm) and 27 clerks from the same factory matched by age. Respiratory function tests were performed on a Monday and Friday of the same week at shift onset, 4 h later and at shift end. The subjects under study were asymptomatic for asthma. The two groups had quite similar spirometric values with minimal functional impairment. A statistical analysis was carried out by Student's t-test for matched pairs and two-way analysis of variance (ANOVA), in order to take into account both occupational exposure and smoking habits. No significant differences between the two groups were observed with paired t-tests in the respiratory parameter trend during both the Monday and Friday work shift. Nor were differences observed within the two groups when Friday's and Monday's results were compared. No significant differences between the two groups were found in paired comparisons between Friday and Monday for respiratory parameters. ANOVA demonstrated that the FEV1 and FEF25-75 reduction present on Friday, when compared to Monday, was related to smoking and not to occupational exposure. In conclusion our findings showed no short-term respiratory changes in subjects exposed to low MDI concentration.