Serum ferritin, transferrin, iron and haptoglobin have been investigated in a longitudinal study in 18 patients hospitalized for various acute infections. Within a couple of days after the onset of an infection, a rise in serum ferritin was seen, the magnitude of which was not dependent on the type of infection (bacterial or viral). The serum ferritin level remained elevated for several weeks in some patients, and 7 out of the 18 patients still had abnormally high values 5 weeks after the onset of illness. The mean curves for serum ferritin and the acute phase reactant haptoglobin were parallel. Possible mechanisms causing the elevation in serum ferritin are discussed.