To determine the impact on iron status of introducing cow's milk (CM) into the diet during the second 6 months of life, nutrient intake was assessed and iron status measured in 100 infants. Nutrient intake for 40 of the 45 infants, age 8 to 13 months, fed CM as the primary beverage for at least 3 months prior to the study and for 45 of 55 infants the same age fed a milk-based infant formula (FF) as the primary beverage for at least 3 months were assessed. All infants in the study were healthy, and the majority were taking no medications or supplements other than vitamins or fluoride for 3 weeks prior to the assessment. Blood drawn by peripheral venipuncture was analyzed by Coulter Counter for complete blood count; plasma albumin, iron, ferritin, transferrin saturation, and total iron-binding capacity were measured in all infants. CM-fed infants had significantly lower mean iron and vitamin C intakes, plasma albumin, transferrin saturation, and ferritin than did FF infants. The frequency of low plasma iron, low transferrin saturation, and low plasma ferritin was significantly greater in CM-fed than in FF infants. The percentage of subjects with three or more abnormal iron indices was more than twice as great in CM-fed infants (58%) as in FF infants (23%). Feeding infants iron-fortified formula to 12 months of age appears to deter iron deficiency.