A prospective longitudinal cohort study was performed to assess the prevalence of iron deficiency in European infants at 12 mo of age, and to study the influence of socio-economic status, dietary factors, growth and morbidity on iron status. The cohort consisted of 488 normal term infants from primary healthcare centres in 11 European areas. Assessed were socio-economic variables, dietary intake, anthropometry and morbidity at regular intervals from birth to 12 mo, and haemoglobin, serum ferritin, mean corpuscular volume, transferrin saturation and serum transferrin receptor concentrations at age 12 mo. The prevalence of anaemia was 9.4%, of iron deficiency 7.2%, and of iron deficiency anaemia 2.3%. More than 40% of anaemia was associated with normal iron status and associated with an increased frequency of recent infections. Iron deficiency anaemia was significantly more frequent with low (5.1%) than high socio-economic status (0%). Dietary factors accounted for most of this variation in multiple regression analysis. Early introduction of cows' milk was the strongest negative determinant of iron status. Feeding of iron-fortified formula was the main factor positively influencing iron status. Other dietary factors, including breastfeeding, did not play a significant role as determinants of iron status at age 12 mo. Conclusion. Iron deficiency anaemia is present in 2.3% of 12-mo-old European infants. The prevalence of iron deficiency anaemia varies strongly with socio-economic status. Avoidance of cows' milk feeding during the first year of life is the key measure in the prevention of iron deficiency.