Abstract Background In Finland, a large-scale health and social service reform was implemented in 2023. The structural reform centralised the organising of health and social services from some 200 organisations to 22 wellbeing services counties (WBSC). One goal of the reform was to promote service integration between and within health and social services. The reform offers a unique opportunity for investigating how such change unfolds. This study addresses the integration aspirations of those who are responsible for implementing the change, that is middle managers in WBSCs. We focus on micro and meso levels and explore how the aim of multidisciplinary collaboration is interpreted as part of advancing service integration by middle managers in one newly formed WBSC. Methods The data for this qualitative study were derived from 11 semi-structured interviews with senior and middle managers responsible for various fields of health and social care services in a single WBSC between October 2022 and June 2023. Inductive thematic analysis was applied. Results The managers framed multidisciplinary collaboration very positively and easier to achieve in the new organisation with health and social services united under the same management. The managers acknowledged the need to form collaborative structures (e.g. meetings) and processes at all levels of organisation. Leadership plays a crucial role in creating space for the professionals to form an experience of a shared every-day work. However, previous structures and patterns of work still seemed to limit these endeavours. Tensions between, for example, professional groups or units were left essentially untouched. Multidisciplinary collaboration will serve especially persons with complex and multiple needs and was framed key to cost savings. Conclusions While the structural reform opens a window of opportunity to new collaborative patterns, competing logics and interests as well as power imbalances may hamper the development if not addressed. Key messages • Middle managers had high hopes for multidisciplinary work after the health system reform, including effects on the difficult issues concerning costs and health workforce availability. • While the managers were prepared to promote multidisciplinary work, the prior organisational boundaries and logics hindered integration. Different tensions between stakeholders need more attention.