The ability of infant temperament and maternal sensitivity to predict 24 month behavior problems was examined in a normative sample of 88 mother-infant dyads from Baltimore, Maryland. Mothers completed the Infant Temperament Questionnaire when their children were 4 and 12 months of age; they completed the Child Behavior Checklist when their infants were 24 months of age. A 45-minute free-play interaction was used to evaluate maternal sensitivity when infants were 4 months of age. Externalizing behavior problems at 24 months of age was predicted by difficult infant temperament at 4 months of age and maternal sensitivity. Maternal sensitivity also predicted internalizing, and total behavior problems at 24 months of age. Low levels of maternal sensitivity moderated the relation between 4 month difficult temperament and 24 month total behavior problems. Results indicate that low maternal sensitivity at 4 months of age interacts with difficult infant temperament at 4 months of age to predict high incidences of total behavior problems when children are 24 months of age. This research demonstrates a need to evaluate maternal sensitivity early in a child's life, before the environment has had an opportunity to affect an infant's seemingly innate temperamental tendencies.