Background: Sensitive responsiveness is an important aspect in affect-regulation of people with severe to profound intellectual disabilities. Aim: This randomized controlled trial evaluated the Attune & Stimulate-checklist, a tool for detecting subtle and idiosyncratic communicative behaviours and responding adequately. Methods: Effects on the sensitive responsiveness of professional caregivers and the arousal and valence of adults with severe to profound intellectual disabilities were investigated. Video recordings of 102 interactions were analysed with several observation instruments. Results: Although no significant effect was found on the checklist-suggestions of responsive behaviours (d = 0.33, p = 0.052), the intervention increased caregivers' sensitive responsive and affective behaviour (d = 0.94 - 1.10, p < 0.001) and clients' optimal arousal (d = 0.48, p = 0.019) and interactive engagement (d = 0.40 - 0.48, p = 0.018 - 0.050). Conclusion: This low-intensity intervention had a medium to large immediate effect on the interaction. Future research should investigate medium- and long-term effects.