The relationship between abnormal behavior and poor developmental test performance was analyzed in 68 6- to 24-month-old Guatemalan babies with and without mild iron deficiency anemia. Regardless of age, the 10 anemic infants with abnormal affective responses during developmental testing had very low mental scores (mean = 65.7 +/- 5.2 SEM), while the 18 with normal affect had mental scores (mean = 97.1 +/- 4.5 SEM) which were normal by U.S. standards and comparable to the nonanemic group's scores. Five anemic infants with pervasive behavioral disturbance, who showed abnormal orientation to tasks in addition to disturbed affect, did poorly on the motor test (mean = 59.8 +/- 6.2 SEM). Those anemic infants who were normal in task orientation achieved motor scores similar to those of the nonanemic control group. The observed behavioral disturbances are consistent with biochemical evidence concerning the role of iron in the metabolism of central nervous system neurotransmitters which influence affect and arousal. These results suggest that poor mental developmental test performance in infants with iron deficiency anemia may be mediated by disturbances in affective behavior.