Epidemiological and in vivo studies suggest that inhaled endotoxin may be an important environmental factor associated with the increases in asthma-related morbidity and mortality. Recent studies by our group and others provide a rationale for the hypothesis that airway exposure of atopic asthmatics to both allergen and endotoxin might result in greater inflammatory responses than those observed with either stimulus alone. Moreover, these studies may provide further evidence that concomitant exposure to allergen and endotoxin is an important factor in asthma pathogenesis.