Symptoms of building sickness were studied among 4373 office workers in 42 different office buildings and 47 different ventilation conditions. The overall response rate was 92%. Assessments of buildings were also made. Work-related symptoms were common in all work groups. The mean number per worker (maximum 10) was significantly greater in women than in men (3.66 vs 2.75) and independently greater in clerical or secretarial workers (3.65) than technical or professional workers (2.85) who in turn reported more symptoms than managers (2.50). Overall, buildings with ventilation from local or central induction/fan coil units had more work-related symptoms per worker than buildings with ‘all air’ systems, which in turn had more than naturally or mechanically ventilated buildings. There was, however, wide variation between the building sickness indices of buildings within each ventilation category which was only partially explained by differences in jobs and gender of their workforces. Symptoms increased substantially once the air supply was chilled or humidified. Microbiological contamination of the supply air from chillers as well as humidifiers may account for some of the symptoms found. The commonest work-related symptom was lethargy (57%) closely followed by blocked nose (47%), dry throat (46%) and headache (46%). The least common symptoms were chest tightness and difficulty in breathing (9%); however, the symptoms were more than twice as frequent in buildings with local and centrally supplied induction/fan coil units compared with the naturally ventilated buildings. This study shows that the symptoms of building sickness are widespread and are not related to a few problem buildings, and that design characteristics of the building and its plant are associated with substantial differences in the reported frequencies of symptoms in those working within them.