Introduction There is an urgent need for sustainable thinking and practices in healthcare systems to meet the challenge of climate change (Charlesworth & Jamieson, 2019; Corvalan et al., 2020; Luykx & Voetterl, 2022; Madden et al., 2020). This need is accelerated by the recent energy crisis. According to an international NGO policy paper (Karliner et al., 2019) healthcare institutions are large energy consumers and major emitters. The (mental) health sectors of the Netherlands and Flanders, the northern part of Belgium, also greatly contribute to the global climate crisis. Both regions have per capita emissions (between the 0.50t and 1t) that fall just below the world’s healthcare top emitters. Objectives To evaluate the state of sustainability in Dutch and Flemish mental health institutions (including psychiatric hospitals, rehabilitation centers, and community mental health centers) and assess whether certain differences can be found in the climate policies of these institutions between both regions. Methods Board members of mental health institutions were asked to complete a 20-item online survey in which concrete actions, objectives and ambitions in the field of sustainability were questioned. Frequencies and percentages were calculated for each question. For certain topics chi-squared tests were performed to test differences in sustainability issues addressed in the questionnaire between Dutch and Flemish mental healthcare institutions. Results Survey response rates for Dutch and Flemish mental health institutions were 38% and 20%, respectively. Ninety-five percent and 38% of respectively the Dutch and Flemish institutions fully agreed that sustainability is a very important theme (χ2(1)=2,25, p=0,13). Key focus areas in both regions included sustainable energy transition (with half of the mental health institutions sourcing at least half of their energy via renewable energy resources and technologies) and recycling (almost 80% of the institutions). Statistically significant differences were found ...