In previous studies, infants of depressed mothers have been found to exhibit reduced left frontal brain electrical activity (EEG). The left frontal region has been hypothesized to mediate social approach behaviors and positive affective expression. These findings raise important questions about the cause and nature of atypical EEG patterns in infants of depressed mothers. The present study begins to address some of these questions by examining whether or not variations in patterns of frontal brain activity in infants of depressed and nondepressed mothers are related to variations in infant behavior as observed in naturalistic situations. If such relations exist, are they specific to certain behaviors hypothesized to be mediated by the frontal region (i.e., positive approach behaviors)? Frontal and parietal brain electrical activity was recorded from 14- to 15-month old infants of depressed versus nondepressed mothers during a baseline condition and during conditions designed to elicit interest and positive affect. Infant behavior was observed in naturalistic play conditions, with and without mother, on a separate day from EEG testing. Mothers provided information on infant temperament. Infants of depressed mothers showed less affection and touching of their mothers. For infants of depressed mothers only, reduced left frontal brain activity was found to be related to lower levels of affection toward mother, but not to infant temperament. Furthermore, increased generalized frontal activation was found to be related to higher levels of negative affect, hostility, and tantrums and aggression. Relations between infant brain activity and behavior were not found for parietal EEG activity. These results suggest that infant frontal electrical brain activity is related to variations in infant behavior, especially those involved in positive affiliative behavior and the expression and regulation of negative affect. The nature and cause of atypical patterns of brain activity and question of whether such atypical patterns of frontal brain activity predispose infants to affective disorders in later life are discussed.