Iron status and body iron reserves were examined in a broad spectrum of the Danish population, and sex- and age-related changes determined. Serum ferritin concentration was employed as an indicator of mobilizable body iron stores. The relationship between serum ferritin and histochemical assessment of stainable bone marrow haemosiderin iron was examined in healthy individuals, defining threshold values for serum ferritin indicating exhausted, small, normal, ample, and increased iron stores. The populations examined comprised 7241 randomly selected normal individuals with an age distribution ranging from newborn to 85 years old. The influence of factors having a significant impact on iron balance, e.g., menstruation, pregnancy, parity, and blood donation, was analysed separately. Newborns had high cord serum ferritin levels, which were to a certain extent dependent on the mother's iron status. Newborns of mothers taking iron supplementation during pregnancy had higher cord serum ferritin than newborns of mothers taking a placebo. In children, the serum ferritin level was relatively constant from 3 years of age until adolescence, where the prevalence of exhausted iron stores was 13% in boys and 18% in girls. In postadolescent men, there was a gradual increase in serum ferritin levels until 30 years of age. Subsequently, serum ferritin remained relatively constant until old age. Among 30- to 70-year-old men, 9.4% had ample iron stores. The prevalence of depleted iron stores was 1.4%, and of iron deficiency anaemia 0.24%. In women, serum ferritin levels remained low from adolescence until the menopause. Among 30- to 50-year-old premenopausal women, the prevalence of ample iron stores was 0.49%, whereas 18% had exhausted iron reserves and 2.6% had iron deficiency anaemia. After menopause, serum ferritin gradually rose and approached male levels. Among 60- to 70-year-old postmenopausal women, 3.0% had ample iron stores, 2.3% had depleted stores and none had iron deficiency anaemia. In fertile women, the choice of contraception had a significant influence on the iron loss at menstruation. Hormonal contraception reduced iron loss, whereas the use of intrauterine devices increased iron loss. These effects were reflected in the serum ferritin levels of menstruating women. In a placebo-controlled study of iron supplementation during pregnancy, serum ferritin levels displayed a characteristic fall both in placebo- and iron-treated women. Among placebo-treated pregnant women at term, 19% had depleted iron reserves and 12% had iron deficiency anaemia, and among iron-treated women, 3% had depleted iron stores and none had iron deficiency anaemia. In non-pregnant women, there was a significant inverse relationship between parity and serum ferritin. Blood donation had a marked influence on the serum ferritin level in the adult population. Among Danes, 20-60 years of age, 27% of men, 15% of premenopausal and 10% of postmenopausal women were blood donors. In all three groups, the prevalence of depleted iron depots was higher in donors than in non-donors. Among premenopausal female blood donors, 31.7% had depleted iron reserves and 3.3% iron deficiency anaemia. In general, Danish men and postmenopausal women had a satisfactory iron status. Adolescent Danish girls and premenopausal women had a high prevalence of iron deficiency, which should be taken into consideration when establishing guidelines and recommendations for nutritional iron intake in this section of the population.