Obligate heterozygotes for cystic fibrosis, like the patients themselves, have significantly increased pupillary alpha-adrenergic and cholinergic sensitivity and significantly decreased lymphocyte beta-adrenergic responses. The parents of patients with cystic fibrosis also have a high prevalence of airway reactivity: one third have a fall in forced expiratory volume in one second (FEV1) of more than 20% (PD20 FEV1) to less than 3 mg/ml methacholine, compared with 4% in control subjects (p = 0.01, chi-square). Airway reactivity (PD20 FEV1) is significantly correlated with the lymphocyte adenosine 3':5' cyclic monophosphate response to isoproterenol (Rs = 0.54, p less than 0.05). Abnormal autonomic and airway reactivity cannot be ascribed to medication usage, preexisting pulmonary disease, or atopy in these subjects. We conclude that the autonomic abnormalities in cystic fibrosis are inherited and may contribute to the development and progression of pulmonary disease by predisposing to increased airway reactivity.