Non‐allergic bronchial hyper‐reactivity is a feature of most patients with asthma. We have measured non‐allergic bronchial reactivity to inhaled histamine and methacholine in thirteen asthmatic subjects before and after allergen inhalation in the laboratory. The allergen inhalation produced mild early asthmatic responses (19–40% FEV₁ fall) in all thirteen, additional definite late asthmatic responses (17–29% FEV₁ fall) in four, and equivocal late asthmatic responses (5–11% FEV₁ fall) in five. Following allergen inhalation, non‐allergic bronchial reactivity increased in seven for up to 7 days. The seven included all four with definite late asthmatic responses and three of the five with equivocal late asthmatic responses. We conclude that allergens make asthma worse, partly through non‐allergic mechanisms, and that avoidance of allergens is important in reducing non‐allergic bronchial hyper‐reactivity.