Patients with cystic fibrosis (CF) secrete copious amounts of mucous material which is viscous, tends to accumulate in the respiratory tract and contains larger than normal amounts of sulfate. The present investigation was designed to measure sulfate levels in the serum of patients with cystic fibrosis by ion chromatography of protein-free serum aliquots. The level of inorganic sulfate in the serum of non-cystic fibrosis pediatric patients averaged 0.29 ± 0.03 mmol/l while patients suffering from cystic fibrosis had an average serum sulfate value of 0.27 ± 0.03 mmol/l which was not significantly different from controls. No differences were observed in serum sulfate levels among males and females of either group of patients. There was a tendency for serum sulfate levels to decrease with age, but there was no statistically significant difference in serum sulfate levels between cystic fibrosis patients and normals as a function of age. These findings indicate that the highly sulfated mucoid materials secreted by cystic fibrosis patients are not reflected in abnormal serum sulfate levels. © 1984.