Rationale and Objective: Acute declines in glomerular filtration rate (GFR) occur commonly during intensive BP lowering. Our objective was to determine the relation between acute declines in estimated GFR and patient outcomes. Study Design, Setting, and Participants: Participants from four randomized controlled trials of intensive BP lowering with CKD (MDRD, AASK, SPRINT, and ACCORD). Exposure: Acute declines in estimated GFR (defined as a >15% versus <15% decline in eGFR between baseline and month 4). Outcomes: Risk of kidney replacement therapy (KRT, primary outcome). KRT was defined as need for dialysis or transplant except in the ACCORD trial, where the trial-defined kidney outcome of a composite occurrence of serum creatinine >3.3 mg/dl, kidney failure, or KRT was used. Approach: Multivariable Cox models. Results: We included 4,465 individuals randomly assigned to intensive versus usual BP control who had a total of 350 kidney outcomes and 304 deaths over median follow-up of 21 and 24 months, respectively. Approximately 15% of participants exhibited an acute decline in eGFR; 11.0% in the usual and 17.7% in the intensive BP treatment arm. In adjusted models, compared to a ?15% decline in the usual BP arm, a ?15% decline in the intensive BP control arm was associated with lower risk of the kidney outcome (0.73; 95% CI 0.56-0.96). In contrast, a >15% decline in eGFR was associated with higher risk of the kidney outcome in both the usual (HR 2.48; 95% CI 1.81-3.40) and intensive BP treatment arms (HR 2.04; 95% 1.48-2.81) compared with a ?15% decline in the usual BP arm. Limitations: Observational study, residual confounding. Conclusions: Declines in eGFR >15% in both the usual and intensive BP treatment arms were associated with higher risk of kidney outcomes in comparison with a ?15% decline in the usual BP arm and may be a harbinger of adverse outcomes.