The relationship between foster children and their foster carers comes with many risks and may be very stressful both for parents and children. We developed an intervention (foster family intervention [FFI]) to tackle these risks. The intervention focuses on foster children below the age of 5 years. The objective was to investigate the effects of FFI on the interactions between foster parents and foster children. A randomized control trial was carried out with a sample of 123 preschool aged children (mean age 18.8 months; 51% boys) and their foster carers. A pretest was carried out 6 to 8 weeks after placement and a posttest one half year later. Interactions were videotaped and coded using the Emotional Availability Scales (EAS). Foster carers were asked to fill in the Dutch version of the Parenting Stress Index. Morning and evening samples of children's salivary cortisol were taken. In the posttest, significantly positive effects were found on the following EAS subscales: Sensitivity, Structuring, Nonintrusiveness, and Responsiveness. We found no significant differences on stress levels of foster carers and children (Nijmeegse Ouderlijke Stress Index domains and salivary cortisol). This study shows that the FFI has a significant positive effect on parenting skills as measured with EAS and on Responsiveness of the foster child. Findings are discussed in terms of impact and significance relating to methodology and design of the study and to clinical relevance.