INTRODUCTION:In 2012, the Veterans Health Administration (VHA) implemented guidelines seeking to reduce PSA-based screening for prostate cancer in men aged 75 years and older. OBJECTIVES: To reduce the use of inappropriate PSAbased prostate cancer screening among men aged 75 and over. SETTING: The Veterans Affairs Greater Los Angeles Healthcare System (VA GLA) PROGRAM DESCRIPTION: We developed a highly specific computerized clinical decision support (CCDS) alert to remind providers, at the moment of PSA screening order entry, of the current guidelines and institutional policy. We implemented the tool in a prospective interrupted time series study design over 15 months, and compared the trends in monthly PSA screening rate at baseline to the CCDS on and off periods of the intervention. RESULTS: A total of 30,150 men were at risk, or eligible, for screening, and 2,001 men were screened. The mean monthly screening rate during the 15-month baseline period was 8.3 %, and during the 15-month intervention period, was 4.6 %. The screening rate declined by 38 % during the baseline period and by 40 % and 30 %, respectively, during the two periods when the CCDS tool was turned on. The screening rate ratios for the baseline and two periods when the CCDS tool was on were 0.97, 0.78, and 0.90, respectively, with a significant difference between baseline and the first CCDS-on period (p<0.0001), and a trend toward a difference between baseline and the second CCDS-on period (p=0.056). CONCLUSION: Implementation of a highly specific CCDS tool alone significantly reduced inappropriate PSA screening in men aged 75 years and older in a reproducible fashion. With this simple intervention, evidence-based guidelines were brought to bear at the point of care, precisely for the patients and providers for whom they were most helpful, resulting in more appropriate use of medical resources.