OBJECTIVES This study examined effects of iron deficiency anemia (IDA) on specific domains of infant cognitive function and the role of IDA-related socioemotional deficits in mediating and/or moderating these effects. METHODS Infants were recruited during routine 9-month visits to an inner-city clinic. IDA was defined as hemoglobin level <110 g/L with > or =2 abnormal iron deficiency indicators (mean corpuscular volume, red cell distribution width, zinc protoporphyrin, transferrin saturation, and ferritin). At 9 and 12 months, the Fagan Test of Infant Intelligence (FTII); A-not-B task; Emotionality, Activity, and Sociability Temperament Survey; and Behavior Rating Scale were administered. Analyses were adjusted for potential confounders, including age and sociodemographic variables. RESULTS Twenty-eight infants met criteria for IDA, 28 had nonanemic iron deficiency (NA ID) and 21 had iron sufficiency (IS). There was a linear effect for object permanence at 9 months: infants with IDA were least likely to exhibit object permanence, IS most likely, and NA ID intermediate. Infants with IDA and those with hemoglobin level < or =105 g/L showed poorer recognition memory on the FTII than infants without IDA. The Behavior Rating Scale orientation/engagement measure partially mediated these effects. Stronger effects of IDA on these outcomes were seen in infants who scored more poorly on the socioemotional measures. CONCLUSIONS These data indicate poorer object permanence and short-term memory encoding and/or retrieval in infants with IDA at 9 months. These cognitive effects were attributable, in part, to IDA-related deficits in socioemotional function. Children with poor socioemotional performance seem to be more vulnerable to the effects of IDA on cognitive function.