Cross-sectional surveys of Norwegian populations indicate that 14-17 per cent of the children below the age of 15 years have had an allergic disease. The corresponding figure in adults is 37 per cent. Among children allergic diseases tend to be more frequent in boys than in girls. In adults there is no significant sex-difference regarding the prevalences of hay fever and obstructive lung disease, while eczema and urticaria are more frequent in women than in men. There is an association of smoking, occupational airborne exposure and degree of industrialization to the prevalences of allergic diseases. There are indications of an increase in occurrence of allergic diseases. The need for standardized operative definitions of these diseases is emphasized.