BACKGROUND: Long-term effects of behavioural weight loss interventions on diabetes complications are unknown. In a secondary analysis of the Look AHEAD (Action for Health in Diabetes) multicentre randomised clinical trial, we assessed whether an intensive lifestyle intervention (ILI) affects the development of nephropathy in people with type 2 diabetes. METHODS: Overweight or obese people aged 45-76 years with type 2 diabetes were randomly assigned (1:1) to ILI or to a diabetes support and education (DSE) group by a central web-based data management system, stratified by clinical centre and blocked with random block sizes. The ILI was designed to achieve and maintain weight loss through reduced caloric consumption and increased physical activity. The interventions were terminated early because of absence of effect on the primary outcome of cardiovascular disease events in the main Look AHEAD trial. Albuminuria and estimated glomerular filtration rate were prespecified as two of many other outcomes and were assessed from baseline until the interventions ended. They were combined post hoc to define the main outcome for this report, very-high-risk chronic kidney disease (CKD), based on the 2013 Kidney Disease Improving Global Outcomes (KDIGO) classification. Outcomes assessors and laboratory staff were masked to treatment, but participants and interventionists were not masked. Time-to-event data were analysed by intention to treat by the Kaplan-Meier method and proportional hazards models. The Look AHEAD trial is registered with ClinicalTrials.gov, NCT00017953. FINDINGS: Of the 5145 participants randomly assigned in the Look AHEAD trial (2570 to ILI and 2575 to DSE), analyses for very-high-risk CKD were done in 2423 (94%) of patients in the ILI group and 2408 (94%) of those in the DSE group. After a median of 8·0 years (IQR 7·9-9·9) of follow-up, the incidence of very-high-risk CKD was lower in the ILI group than in the DSE group, with incidence rates of 0·91 cases per 100 person-years in the DSE group and 0·63 per 100 person-years in the ILI group (difference 0·27 cases per 100 person-years, hazard ratio 0·69, 95% CI 0·55-0·87; p=0·0016). This effect was partly attributable to reductions in bodyweight, HbA1c, and systolic blood pressure. There were no safety concerns regarding kidney-related adverse events. INTERPRETATION: Weight loss should be considered as an adjunct to medical treatments to prevent or delay progression of CKD in overweight or obese people with type 2 diabetes. FUNDING: National Institute of Diabetes and Digestive and Kidney Diseases.