Background <p>Diagnosis of rare chronic kidney disease (CKD) can be difficult with conventional diagnostic workup. When no diagnosis is identified, this is termed CKD of unknown aetiology (CKDUA). There are multiple benefits in obtaining a diagnosis in these cases. The advent and increasing availability of genetic testing in recent years has been a welcome additional diagnostic tool. In this study we aimed to determine the demographics and kidney related outcomes for a cohort of patients with CKDUA within the Salford Kidney Study (SKS).</p> Methods <p>The SKS is a single-centre, ongoing, prospective, observational cohort study of adult patients referred to the renal service at Salford Royal Hospital, UK. Within the SKS there are 398 patients with CKDUA. A group with diabetic kidney disease (DKD) was used as a comparator. An analysis was performed comparing these two groups with a particular focus on their demographics and kidney related outcomes.</p> Results <p>Median age of the CKDUA cohort was 71.4 years, with 60.1% male and 96.2% White. There was advanced CKD at presentation (median eGFR 30&#xa0;ml/min/1.73m²), with a 5-year mortality of 27.6%. When compared with the DKD cohort, the CKDUA cohort were older (71.4 years vs. 67.3 years, <i>p</i> &lt; 0.001), had more equal sex distribution (male 60.1% vs. 68%, <i>p</i> &lt; 0.009), had less advanced renal disease at presentation (eGFR 30&#xa0;ml/min/1.73m² vs. 26&#xa0;ml/min/1.73m², <i>p</i> &lt; 0.001), and were less likely to progress to renal replacement therapy (14.1% vs. 28.2%, <i>p</i> &lt; 0.001).</p> Conclusions <p>CKDUA is difficult to manage: other than general supportive measures there is little else that can be offered. This study reports on a real-world cohort of patients with CKDUA and identifies the surprising finding that in comparison to a DKD cohort their outcomes are improved, with a significantly lower rate of progression to RRT.</p> Clinical trial number <p>Not applicable.</p>