Background Obesity increases risk of hypertension and diabetes, the leading causes of end-stage renal disease. The effect of obesity on kidney function decline in stable post-myocardial infarction patients is poorly documented. This relation was investigated in a large cohort of older post-myocardial infarction patients. Design Data were analysed from 2410 post-myocardial infarction patients in the Alpha Omega Trial, aged 60–80 years receiving optimal pharmacotherapy treatment (79% men, 18% diabetes). Methods Cystatin C based estimated glomerular filtration rate (eGFR_cysC) was calculated at baseline and after 41 months, using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. Obesity was defined as body mass index ≥ 30 kg/m² and high waist circumference as ≥102 and ≥88 cm for men and women. The relation between body mass index, waist circumference and annual eGFR_cysC decline was evaluated by linear regression. Results At baseline, mean (standard deviation) eGFR_cysC was 81.5 (19.6) ml/min/1.73 m², 23% of all patients were obese. After multivariable adjustment, the annual mean (95% confidence interval) eGFR_cysC decline in men and women was –1.45 (–1.59 to –1.31) and –0.92 (–1.20 to –0.63) ml/min/1.73 m², respectively ( p = 0.001). Obese versus non-obese patients and patients with high versus normal waist circumference experienced greater annual eGFR_cysC decline. Men and women showed an additional annual eGFR_cysC decline of –0.35 (–0.56 to –0.14) and –0.21 (–0.55 to 0.14) ml/min/1.73 m² per 5 kg/m² body mass index increment ( p for interaction 0.3). Conclusions High compared to normal body mass index or waist circumference were associated with more rapid kidney function decline in older stable post-myocardial infarction patients receiving optimal drug therapy.