The merits of using financial incentives to improve clinical quality have much appeal, yet few studies have rigorously assessed the potential benefits. The uncertainty surrounding assessments of quality can lead to poor policy decisions, possibly resulting in increased cost with little or no quality improvement, or missed opportunities to improve care. We conducted an experiment involving physicians in thirty Philippine hospitals that overcomes many of the limitations of previous studies. We measured clinical performance and then examined whether modest bonuses equal to about 5 percent of a physician's salary, as well as system-level incentives that increased compensation to hospitals and across groups of physicians, led to improvements in the quality of care. We found that both the bonus and system-level incentives improved scores in a quality measurement system used in our study by ten percentage points. Our findings suggest that when careful measurement is combined with the types of incentives we studied, there may be a larger impact on quality than previously recognized.