Cough, dyspnea, and chest pain are symptoms common to many cardiopulmonary diseases. A comprehensive evaluation, including a history, physical examination, ECG, chest roentgenogram, and pulmonary function studies, will often yield a specific diagnosis. However, when these symptoms are intermittent, as they often are in patients with bronchial asthma, the diagnosis may not be apparent. If asthma is thought to be a diagnostic possibility, a bronchial inhalation challenge should be used to demonstrate bronchial hyperreactivity, the hallmark of asthma. The methacholine chloride inhalation challenge is a simple and useful laboratory test to diagnose bronchial hyperreactivity. We describe eight patients with a variety of clinical symptoms to demonstrate the usefulness of this test. Patients with unexplained respiratory symptoms should be considered for bronchial inhalation challenge before proceeding to more invasive diagnostic procedures.