This paper is a report of a study to develop milieu therapists' acknowledging communication in their relationships with patients. Gunderson's therapeutic processes in milieu therapy have come into use in a broad range of mental health contexts in many countries. Research in nursing indicates that validation needs a more concrete development for use in clinical work. Schibbye's theory, 'Intersubjective relational understanding', formed the theoretical foundation for a participatory action research project in 2004-2005. The data comprised the researcher's process notes written during participation in the group of group leaders every second week over a period of 18 months, clinical narratives presented by participants in the same group, and eight qualitative interviews of members of the reflection group. The core concept in acknowledging communication, mutuality, was described as inter-subjective sharing of feelings and beliefs in a respectful way. Participants presented their process of development as a movement from knowing what was best for the patient (acknowledging patients as competent persons, a milieu-therapy culture based on conformity), to appreciating diversity and stubborn talk, to reflective wondering questions. Misunderstanding of acknowledgement occurred, for instance, in the form of always being supportive and affirmative towards patients. The concrete approaches in acknowledging communication presented in this article could be a fruitful basis for educating in and developing milieu therapy, both for nursing and in a multi-professional approach in clinical practice and educational institutions. Future research should focus on broader development of various areas of acknowledging communication in practice, and should also include patients' experiences of such approaches.