Introduction: Although some interventions on non-communicable diseases (NCDs) have been evaluated in sub-Saharan Africa, little is known about their medium to long term sustainability beyond the end of funding. A cluster randomised trial of a health system intervention to improve NCD care was conducted between 2013 and 2016 in 38 primary care and 6 referral health facilities (HFs) each in Tanzania and Uganda, focusing on hypertension (HT) and type-2 diabetes mellitus(DM). It involved a combination intervention which the trial showed to be highly effective in improving HT AND DM service readiness and quality of care. This PhD research aimed to assess the sustainability of the intervention at 22 HFs (19 lower level units that constituted the original intervention arm and 3 referral facilities that also received the intervention) in Uganda, 4 years after the end of the trial. Methods: This PhD study compared i) the health facility performance (FPS) in terms of health worker knowledge, service availability and readiness (SAR), using a modified WHO SARA tool and, ii) the patient quality-of-care and experience (QoCE) according to national guidelines using a previously validated tool. Cross-sectional data from the original trial (2016) and this study (2020) were compared. Using a pair-matched approach, FPS and QoCE summary scores were compared. Linear regression and random effects Tobit regression models were also analysed. Additionally, iii) the current capacity and practice to sustain ongoing intervention activities for HT and DM care in these facilities was also assessed in 2020. Through a cross sectional survey, 4 pre-defined domains (i.e., cognitive participation, coherence, collective action, and reflexive monitoring were examined with regards to health worker (HW) normalization and 8 pre￾defined domains for intervention sustainability (i.e., organisational capacity, local environment, funding stability, partnerships, communication, evaluation, adaptation, and strategic planning), using the normalisation tool and the program sustainability tool (PSAT). Results: The mean aggregate facility performance (FPS) in 2020 was lower than in 2016: 70.2 (95%CI= 66.0-74.5) vs. 74.8 (95%CI=71.3-78.3) respectively, with no evidence of a significant difference (p=0.18). Mean scores declined in 4 of 5 SAR elements. Only the availability of guidelines and quality of records showed some improvement [9.1 (95%CI: 8.2,9.9) (2016) vs 9.7 (95%CI: 9.5,10.6 (2020)]. No exposure independently predicted FPS although patient club functionality was very weakly associated (p=0.09). QoCE declined slightly to 8.7 (95%CI=8.4-9.1) in 2020 vs 9.5 (95%CI=9.1-9.9) in 2016 (p=0.02) while the proportion of patients receiving adequate quality care also declined slightly to 88.2% from 98.5% respectively, and with no statistical difference (p=0.20). Only the parent district weakly predicted QoCE (p=0.05). Overall normalization strength was adequate at 4.0 (IQR: 3.8, 4.2) of a possible 5 with no evidence of association with HF level (p=0.40); cognitive participation (buy-in) and reflexive monitoring (appraisal) were strongest at >4 across all HF levels. All HF levels were also weak (<4) on collective action (teamwork) and coherence (sense-making); Only collective action differed by level (p<0.002). Overall intervention sustainability was suboptimal at 3.1 [IQR 1.9,4.1]) of a possible 7 with weak scores on funding stability (2.0), supportive partnerships (2.2), and strategic planning (2.6). Domain differences by facility level were significant for environmental support (p=0.02)and capacity in organisation (p=0.01). Adequate strength at a mean cut-off of 5 did not differ by facility level for any domain. Conclusions: Four years after the end of research-related support, both service availability and readiness and quality of HT/DM care were surprisingly well preserved. Practice-dependent intervention elements e.g., local organisational context, HW knowledge or dedication (buy-in) were sustained, but external elements e.g., new funding support or attracting new partners to sustain intervention efforts were not sustained. Sustainability or durability of an HT AND DM intervention in similar primary care settings may remain achievable even with the funding instability and logistical challenges following a research trial’s end. Earlier on in the intervention process, health managers and implementors should plan how to sustain any achievements.