To test whether eosinophil recruitment after pulmonary allergen challenge is associated with interleukin (IL)‐5 in patients with asthma, we performed segmental bronchoprovocation (SBP) with saline, and with low and high dosages of ragweed extract in six patients with allergic asthma. Bronchoalveolar lavage (BAL) of the challenged segments was performed 5 min after challenge (immediate BAL fluid) and repeated 24 h later (late BAL fluid). Allergen challenge resulted in recruitment of eosinophils, and increased levels of eosinophil‐active cytokines. A bioassay showed the predominant eosinophil‐active cytokine in the late BAL fluids to be IL‐5. Analysis of the late BAL fluids revealed that IL‐5 levels correlated with the numbers of eosinophils and lymphocytes. This study provides evidence that IL‐5 is a critical cytokine associated with eosinophil and lymphocyte recruitment into the airways of patients with asthma following exposure to allergen.