Therapy with synthetic ACTH (zinc tetracosactide) in children affected by epileptic encephalopathy is often associated with a large number of infectious complications. We studied the phagocytic activity of polymorphonuclear leucocytes (PMN) in 9 children with West or Lennox-Gastaut syndrome, measuring PMN superoxide anion production during the phagocytosis of particles of Zymosan and after phorbol myristate acetate (PMA) stimulation. The test was performed before, during and after therapy with zinc tetracosactide (0.02 mg/kg/day for 15 days). At the same time plasma immunoglobulins, C3, C4, C3 activator and cortisol were determined. During treatment PMN phagocytic function was significantly reduced but returned to normal levels after suspension of therapy. The other hematological parameters considered remained within the normal range. During the follow-up of the patients we observed 15 infectious episodes (3 mucocutaneous candidiasis, 2 enterocolitis, 4 urinary tract infections, 1 otitis media, 3 bronchiolitis, 2 pneumonia). One of the patients died of a bilateral pneumonia. Three children were treated with ACTH on alternating days. In these patients PMN phagocytic activity was less impaired and 2 infectious episodes rapidly resolved. Alternate day ACTH therapy seems to be preferable.