Within the European research project HOPE, 64 office buildings were audited regarding in-door environment quality and building characteristics. Of those buildings, two groups are se-lected: 21 buildings with low prevalence of the sick building syndrome, and 21 buildings with the largest prevalence. However, none of these buildings was considered a priori as sick. Both groups are similar for age of occupants, type of work, ownership, external environment, outdoor temperature, orientation, and smoking policy. This study shows that large or tall buildings with low ceilings and high occupancy ratio are more likely to present a high build-ing-related symptom index (BSI). Noise sources may also influence the BSI, but no clear rela-tionship between pollution and BSI was found. Finally buildings with mechanical ventilation systems were shown to have on average a higher BSI, but there are buildings where the me-chanical ventilations system is well designed and maintained, which present a low BSI.