The constant increase in asthma and atopy prevalences--despite improved treatment and knowledge of many aspects of the diseases--has raised growing concern. Accumulating evidence suggests that these increases in atopic diseases are largely attributable to environmental and lifestyle factors, and the lack of systemic childhood infections has in many studies emerged as a major factor. In addition to current high standards of hygiene and the lack or scarcity of such infections, another factor characteristic of our present-day lives could be involved. This review briefly outlines the possibility that prolonged maternal stress associated with sustained excessive cortisol secretion could affect the developing immune system--especially T(H)1/T(H)2 cell differentiation--and further increase the susceptibility to asthma and atopy in genetically predisposed individuals. This hypothesis is critically evaluated in the light of current knowledge.