Three patients with X-linked chronic granulomatous disease of childhood (CGD) were treated with a single daily dose of 1 g of vitamin C over a period of 8 months. Prior to this clinical trial, isolated leucocytes from patients and normals were incubated with different concentrations of ascorbate and intracellular killing activity was investigated. Contrary to previous reports, there was no improvement of polymorphonuclear (PMN) intracellular killing activity after oral administration of ascorbate, nor could in vitro ascorbate correct defective, or enhance, killing activity of normal PMNs.