The sharp increase in the prevalence of asthma over the past three decades suggests an important contribution of environmental factors in the generation of this disease, and compels a search for molecular pathways by which such factors could facilitate Th2 immune-inflammatory airway responses; granulocyte-macrophage colony-stimulating factor (GM-CSF) might be one such signal. In this review, we appraise the evidence with respect to the presence of GM-CSF in asthma, the roles played by GM-CSF in these immune responses and environmental triggers that can induce GM-CSF expression. Further, we propose a paradigm that unites these divergent observations, and postulate that GM-CSF produced in response to environmental agents can establish an airway microenvironment that promotes the initiation, influences the evolution and supports the maintenance of an aeroallergen-specific adaptive Th2 immune response.