Importance: Limited evidence supports kidney dysfunction as an indication for parathyroidectomy in asymptomatic primary hyperparathyroidism (PHPT). Objective: To investigate the natural history of kidney function in PHPT and whether parathyroidectomy alters renal outcomes. Design: Matched control study. Setting: A vertically integrated health care system serving 4.6 million patients in Southern California. Participants: 6058 subjects with PHPT and 16 388 matched controls, studied from 2000 to 2016. Exposures: Biochemically confirmed PHPT with varying serum calcium levels. Main outcomes: Estimated glomerular filtration rate (eGFR) trajectories were compared over 10 years, with cases subdivided by severity of hypercalcemia: serum calcium 2.62-2.74 mmol/L (10.5-11 mg/dL), 2.75-2.87 (11.1-11.5), 2.88-2.99 (11.6-12), and >2.99 (>12). Interrupted time series analysis was conducted among propensity-score-matched PHPT patients with and without parathyroidectomy to compare eGFR trajectories postoperatively. Results: Modest rates of eGFR decline were observed in PHPT patients with serum calcium 2.62-2.74 mmol/L (-1.0 mL/min/1.73 m²/year) and 2.75-2.87 mmol/L (-1.1 mL/min/1.73 m²/year), comprising 56% and 28% of cases, respectively. Compared with the control rate of -1.0 mL/min/1.73 m²/year, accelerated rates of eGFR decline were observed in patients with serum calcium 2.88-2.99 mmol/L (-1.5 mL/min/1.73 m²/year, P2.99 mmol/L (-2.1 mL/min/1.73 m²/year, P2.87 mmol/L exhibited mitigation of eGFR decline after parathyroidectomy (-2.0 [95% CI: -2.6, -1.5] to -0.9 [95% CI: -1.5, 0.4] mL/min/1.73 m²/year). Conclusions and relevance: Compared with matched controls, accelerated eGFR decline was observed in the minority of PHPT patients with serum calcium >2.87 mmol/L (11.5 mg/dL). Parathyroidectomy was associated with mitigation of eGFR decline in patients with serum calcium >2.87 mmol/L. [ABSTRACT FROM AUTHOR]