With a large and diverse sample of children from the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development, the role of infant-mother attachment security as a protective factor against the development of children's anxious and aggressive behaviors at first grade was examined. When child's sex, family income, maternal sensitivity, and prior levels of anxiety and aggression were controlled for, attachment security at 15 months of age was found to moderate the effects of negative life events families experienced, thereby protecting children from experiencing symptoms of anxiety, but not aggression, at 4.5 years of age. Children classified as insecurely attached at 15 months of age who experienced many stressful life events exhibited more anxiety symptoms in first grade than children classified as securely attached who similarly experienced many negative life events. These findings are interpreted within attachment theory, which predicts that early attachment security has a unique role in children's anxiety experiences later in childhood.