The significant growth of Baitul Mal wat Tamwil institutions (BMTIs), a renowned Indonesian Islamic microfinance institution, has planted a prominent position in the industry, albeit their accountability has been questioned. Thus, this study analysed three objectives concerning the BMTIs’ accountability issue by investigating their current financial reporting practices, as higher disclosure increases their prospects. Notably, this study is among the few studies on the accountability of BMTIs in Indonesia and internationally. The empirical data were obtained from 206 annual reports of BMTIs throughout 2012-2017, used to examine the disclosure score derived from the disclosure index, developed according to the current accounting regulations. The findings showed the extent and quality of BMTIs obtained through annual report data, in which the mean score for the extent of disclosure was at the medium level (49.56 per cent), while the quality of disclosure was at a lower level (31.81 per cent). Based on the four dimensions of annual reports, the social disclosure scored higher than the financial disclosure, while the extent of Shariah compliance disclosure exhibited the lowest score (37.6 per cent). Second, since BMTIs have different capacities, this study examined the differences in the extent and quality of financial and non-financial disclosures between micro, small, and medium BMTIs. The findings showed differences in the extent and quality of financial and non-financial disclosure between micro, small, and medium BMTIs. The third objective aimed to fill the research gap in Islamic social capital (ISC) applied to BMTI. The types of ISC information are considered essential to disclose in BMTIs’ annual reports. Ultimately, this crucial information provides empirical evidence for the stakeholders. Subsequently, questionnaires were sent to 117 participants to explore the stakeholders’ perceptions of the importance of disclosing Islamic social capital information in the annual reports of BMTIs. The result was then used for factor analysis, examining 36 proposed ISC information disclosed in the annual reports. Furthermore, the study highlighted that it is presumptively critical to disclose ISC information in the annual reports, which are valued drivers of BMTIs’ success. Most respondents perceived ISC components and the crucial information as essential in increasing its accountability level towards the community. Interviews further strengthened the data to complement the view on the importance of ISC information in annual reports of BMTIs, which its stakeholders agreed. BMTIs should be accountable for Shariah compliance and consider halal, haram, Toyyib, and religious activities as part of the belief system of the members and communities. Furthermore, BMTIs should provide significant attention to ISC to potentially strengthen the institution’s capacity and increase transparency by disclosing ISC information. Ultimately, this idea can enhance the members' and non-members' contribution to BMTIs. This study contributes to the understanding of the importance of social capital disclosure in BMTI annual reports. Overall, the findings contribute towards strengthening BMTIs for better accountability and sustainability of BMTIs in Indonesia.