This was a study of the relationship between mechanical ventilation and sick business syndrome in an office building with 1250 employees, where workers blamed the mechanical ventilation and indoor air quality for causing typical SBS symptoms such as nasal, eye, skin and mucous membrane irritations, lethargy and headaches). The building's mechanical ventilation rates were high. A controlled experiment to test the hypothesis that mechanical ventilation causes SBS was made by switching off the ventilation in part of the building while reducing the ventilation rate by 75% and 60% in two other areas while leaving one part unaltered as a control. The reduction of the ventilation rate did not decrease the symptoms; in fact, it caused a slight but statistically significant increase. In the cross sectional analysis of the baseline data the SBS symptoms did not correlate significantly with the ventilation rate. In the linear regression model, a positive correlation was shown between temperatures above 22 C and the incidence of symptoms. Environmental tobacco smoke increased the occurrence of SBS symptoms among workers. Women reported more symptoms than men. Further research showed that a prior history of allergic diseases and dissatisfaction with the social atmosphere at work were significant determinants of SBS symptoms.