Purpose We investigated age related changes in urodynamic parameters in 2 large cohorts of women planning stress urinary incontinence surgery. Materials and Methods Using a standardized protocol we obtained urodynamic parameters for participants in SISTEr (Stress Incontinence Surgical Treatment Efficacy Trial) and TOMUS (Trial of Mid-Urethral Slings) undergoing baseline noninvasive flow followed by filling cystometrogram and pressure flow study. The bladder contractility index (defined as detrusor pressure at maximum flow + 5 × maximum flow) and detrusor hypocontractility (defined as detrusor pressure at maximum flow less than 10 cm H2O) were also characterized. Patients excluded from analysis had undergone prior stress urinary incontinence surgery or had prolapse stage greater than II. Propensity score analysis controlled for the potential bias of combining participants from 2 clinical trials. Linear and logistic regression analysis adjusting for propensity score quintile was done to assess the association of age and an age cutoff (less than 65 vs 65 or greater years) with urodynamic parameters. Results A total of 945 women (468 in SISTEr and 477 in TOMUS) were included in analysis. Mean age was 50 years in SISTEr (range 27 to 75) and 51 years (range 24 to 82) in TOMUS. Noninvasive maximum urinary flow decreased significantly with age (26.2 vs 22 ml per second, p = 0.002). Noninvasive flow voiding time increased 2.7 seconds for each 10-year age increment and detrusor pressure at maximum flow decreased 2.1 cm H2O for each 10-year increase in age (each p = 0.003). Hypocontractility was more likely in women 65 years old or older (OR 2.89, 95% CI 1.59, 5.27). The bladder contractility index was inversely related to age, decreasing a mean ± SD of 7.68 ± 1.96 cm H2O for each 10-year age increase (p <0.001). Conclusions In these 2 cohorts the observed changes in voiding parameters suggest that detrusor contractility and efficiency decrease with age. © 2014 by American Urological Association Education and Research, Inc.