OBJECTIVES:In the past three decades, many “volume-outcome” studies have supported the hypothesis that higher volume was associated with better outcomes. However, most studies were cross-sectional based, thus can only demonstrate the association between volume and outcome, but not the effect of volume on outcome. This study therefore aimed to test the hypothesis that practice-makes-perfect by using longitudinal data to examine how the accumulated experience of Coronary Artery Bypass Graft Surgery (CABG) operation done by junior surgeons (those physicians newly started to file the procedure) may affect patients’ health outcomes. METHODS:Taiwan has implemented the Naitonal Health Insurance since 1995 and can provide complete data of services provided by each physician, therefore we applied for the 1997-2005 National Health Insurance Database from the National Health Research Institute. Patients received CABGs by junior surgeon between 2000 and June 2005 were identified. Major health outcomes included in-hospital mortality rate, mortality rate during the period from admission to discharge, mortality rate during the period from admission to 30 days after discharge. Generalized Estimating Equations (GEE) were used to estimate the effect of volume on outcome after adjusting for selected characteristics of patients, surgeons, and hospitals. RESULTS:Sixty-eight junior surgeons performed CABGs for 3063 patients during 2000 to June 2005 in Taiwan. All health outcomes were significantly better among patients receiving CABGs as junior surgeons’ experience accumulated. Sensitivity analyses revealed the same results. CONCLUSIONS:As junior surgeons’ CABGs experience accumulated, patients’ outcomes tend to become better. The finding supports the hypothesis that "practice-makes-perfect" in the case of junior surgeons performing CABGs in Taiwan.