Background: Children with Reactive Attachment Disorder (RAD) have serious socio-behavioral problems and often rely on socially abnormal, aggressive, and manipulative forms of communication. Little is known, however, about the influence of teachers on the socio-behavioral development of children with symptoms of RAD. Aims: This longitudinal study examined the influence of teacher sensitivity on the socio-behavioral development of children with symptoms of RAD across one school year. Method: The sample included 85 Belgian children and 70 teachers from special education schools. In the previous school year, teachers rated Inhibited and Disinhibited RAD symptoms. In the next school year, teacher Sensitivity was observed in interactions with individual children in the first trimester. Teacher-rated Overt aggression, Relational aggression, and Prosocial behavior was assessed in the first, second, and third trimester. Results: We found no effects of Sensitivity on Prosocial behavior. Also, no effects were found for children with Disinhibited RAD symptoms. For children with Inhibited RAD symptoms, increases in Overt and Relational aggression were observed when Sensitivity was low, whereas decreases were observed when Sensitivity was high. Conclusions and implications: The results suggest that teacher sensitivity is associated with the socio-behavioral development of children with Inhibited RAD symptoms but not with the socio-behavioral development of children with Disinhibited RAD symptoms. What this paper adds: Children with Reactive Attachment Disorder (RAD) exhibit socio-behavioral problems that hinder their school adjustment. These socio-behavioral problems appear relatively stable and it is not known what influence special education teachers might have on the development of these problems across a school year. This study suggests that teacher sensitivity is associated with changes in the socio-behavioral development of children with Inhibited RAD symptoms. Whereas high sensitivity was associated with improvements, low sensitivity appeared to exaggerate the socio-behavioral problems of these children. As children with Inhibited RAD symptoms have difficulties communicating their needs and wishes in socially adaptive ways, it may not be easy for teachers to understand these children. Teachers may misinterpret a child's behavior and consequently will fail to respond to the child's underlying needs. This may reinforce the child's socio-behavioral problems and increase the child's reliance on egocentric and aggressive means in interactions with others. This study therefore highlights the need to support teachers in interactions with children with Inhibited RAD symptoms in order to help them understand how the children's observable behaviors in the classroom may convey their underlying socio-emotional needs and how they can respond to these needs. Importantly, teacher sensitivity was not associated with the socio-behavioral development of children with Disinhibited RAD symptoms (e.g., indiscriminate friendliness). Consistent with previous research, this study suggests that children with Inhibited RAD symptoms are more susceptible to the quality of the caregiving environment than children with Disinhibited RAD symptoms and extends this finding to the school context. (C) 2016 Elsevier Ltd. All rights reserved.