Objectives: To determine the effects of dietary, physiological or environmental factors on body iron levels in infants aged 4±18 months. Design: The daily iron intake of the infants was measured from a diet history obtained by interview using a standardised question sheet, previously validated against weighed intake (minimum 3 days) in an independent sample of 8 and 18 month old infants. Capillary blood samples were analyzed for haemoglobin, mean cell volume, haematocrit, zinc protoporphyrin and plasma ferritin concentration. Ferritin values were log-transformed prior to analysis to give a better approximation to the normal distribution and forward stepwise multiple linear regression was carried out using SPSS. Setting: The city of Norwich, UK and some of its suburbs. Subjects: One hundred and eighty-one healthy infants in age groups 4, 8, 12 and 18 months. Results: Main determinants of iron stores in the 4 month old infants were birth weight ( ve (P`0.001)) and body weight (7ve (P`0.005)). In the 8 month old infants intake of cow's milk (7ve (P`0.05)), belonging to a smoking household (7ve (P`0.05)) and quantity of commercial babyfood consumed ( ve (P`0.05)) were signi®cant. In this age group there was a gender effect (girls b boys (P`0.01)) and the gender effect remained at 12 months (girls b boys (P`0.05)), but at 18 months only non-haem iron intake was a signi®cant factor (7ve (P`0.05)). Conclusions: At 4 months of age birth weight and body weight exert the greatest in¯uence on iron stores, whereas by 8 months components of the weaning diet have an effect (commercial babyfood ( ve), cow's milk (7ve)); there is also a gender effect (girls b boys), possibly re¯ecting the different growth rate between boys and girls. At 12 and 18 months the only signi®cant factors are gender (girls b boys) and non-haem iron intake (7ve) respectively. Sponsorship: Ministry of Agriculture, Fisheries and Food and the Biotechnology and Biological Sciences Research Council.