An epidemiological study was carried out from January 15th to March 31st 1976 among 1321 clerical workers of the Administration of the Canton of Geneva. This study was intended to determine whether indoor humidification during winter prevents respiratory infections. The results obtained show that air-conditioning, in spite of an average relative humidity (RH) of 52%, has an unfavourable influence on respiratory infections which are significantly more numerous among people working in air-conditioned offices. Humidifiers did not have a significant influence on the incidence of respiratory diseases, but the trifling difference of average air humidity between offices equipped with humidifiers (RH:33%) and offices without humidifier (RH:30%) shows that these appliances, as at present commonly used, are inefficient. After a review of the literature on the subject, the authors reach the conclusion that no convincing evidence supports the theory of the effectiveness of indoor humidification in the prevention of respiratory diseases.