OBJECTIVE The aim of the study was to identify the demographic and clinical features in an urban cohort of people with type 1 diabetes who developed a ≥50% decline in es-timated glomerular filtration rate (eGFR). RESEARCH DESIGN AND METHODS We evaluated 5,261 people with type 1 diabetes (51% female, 13.4% African Ca-ribbean) with baseline eGFR >45 mL/min/1.73 m 2 between 2004 and 2018. The primary end point was an eGFR decline of ≥50% from baseline with a final eGFR <30 mL/min/1.73 m 2. eGFR was calculated using the Chronic Kidney Disease Epidemiology Collaboration equation. RESULTS Of the cohort, 263 (5%) reached the primary end point. These individuals were more likely to be of African Caribbean ethnicity, be older, have a longer duration of diabetes, have higher systolic blood pressure and HbA 1c, have more prevalent retinopathy, and have higher albuminuria (all P < 0.05). In multivariable Cox regression models, African Caribbean ethnicity emerged as a significant risk factor for the primary end point (hazard ratio 1.57, 95% CI 1.19, 2.08) compared with other ethnicities and independent of established risk factors (P < 0.01). The incidence rate for the primary end point in African Caribbean people was double that in non–African Caribbean people (16 vs. 7.7 per 1000 patient-years, P < 0.001). A similar significant independent impact of African Caribbean ethnicity for secondary end points (≥40% and ≥30% fall in eGFR) was observed. CONCLUSIONS We report a novel observation that African Caribbean ethnicity increased the risk of kidney function loss in people with type 1 diabetes, an effect that was independent of traditional risk factors. Further studies are needed to examine the associated pathophysiology that may explain this observation.