This prospective cohort study sought to identify predictors of functional decline in patients aged 65 years or older who underwent major, nonemergent abdominal or thoracic surgery in our tertiary hospital from 2006 to 2008. We used the Stanford Health Assessment Questionnaire-Disability Index (HAQ-DI) to evaluate functional decline; a 0.1 or greater increase was used to indicate a clinically significant decline. The preoperative Duke Activity Status Index (DASI) and a physical function score (PFS), assessing gait speed, grip strength, balance, and standing speed, were evaluated as predictors of decline. We enrolled 215 patients (71.2 ± 5.2 years; 56.7% female); 204 completed follow-up HAQ assessments (71.1 ± 5.3 years; 57.8% female). A significant number of patients had functional decline out to 1 year. Postoperative HAQ-DI increases of 0.1 or greater occurred in 45.3 per cent at 1 month, 30.1 per cent at 3 months, and 28.3 per cent at 1 year. Preoperative DASI and PFS scores were not predictors of functional decline. Male sex at 1 month (odds ratio [OR], 3.05; 95% confidence interval [CI], 1.41 to 6.85); American Society of Anesthesiologists class (OR, 3.41; 95% CI, 1.31 to 8.86), smoking (OR, 3.15; 95% CI, 1.27 to 7.85), and length of stay (OR, 1.09; 95% CI, 1.01 to 1.16) at 3 months; and cancer diagnosis at 1 year (OR, 2.6; 95% CI, 1.14 to 5.96) were associated with functional decline.