The present investigation is the first longitudinal study of the concentration of iron in breast milk and is based on 229 milk samples obtained from 27 mothers during their period of lactation up to 9 months. The samples were collected at the beginning and at the end of each feed during a 24-h period to reflect as accurately as possible the actual concentration of iron. The median value declined during the course of lactation from 0.6 to 0.3 mg/l with a large range of values. The results indicate that the concentration is lower than is generally stated or is unusually low in Finnish mothers. As a consequence some infants may require iron supplementation during prolonged breast feeding although in general, the high bioavailability of breast milk iron prevents the development of iron deficiency.