The effect of the housing environment on the frequency and duration of acute respiratory diseases was investigated in children aged one to two years. The increase in the number of days of illness correlated with the following factors: position of the flat towards the main road, strong exposure of the house to wind, and number and size of the windows in the children's room, non-adjustibility of the temperature of the radiator, draught in the flat, lacking possibility of cross ventilation in the flat, bathroom and kitchen without windows, and predominant vacuum cleaning in the flat, in contrast to combined wiping and vacuum cleaning. In addition to the factors mentioned above, the morbidity rate was increased by combined window and forced ventilation, in contrast to mere window ventilation and by staying outdoors at the weekend for less than two hours. Additional studies are necessary before a causal relationship can be established.