SummaryThe relationship between the activity of the cystic fibrosis serum ciliary dyskinesia factor, detected by the rabbit tracheal bioassay, and clinical status of the patient has been investigated in children, 1–24 years old, with cystic fibrosis. No significant correlation was found between the amount of serum factor activity and age, clinical status assessed by the Schwachman score, pulmonary function (vital capacity, functional residual capacity, total lung capacity, residual volume, maximum breathing capacity, maximal midinspiratory flow, and maximal midexpiratory flow) or blood gas levels (pCO2, pO2). The activity showed no significant relationship to serum galactosyltransferase activity in children with cystic fibrosis.SpeculationThe substance responsible for the ciliary dyskinesia serum factor activity detected in children with cystic fibrosis may not be directly involved in the clinical syndrome or in its pathophysiology.