The authors evaluated the effectiveness of the Quality Improvement System (QIS) developed and implemented by Children's Services Council of Palm Beach County (Florida) as a voluntary initiative to improve the quality of childcare and education. They adopted a growth model approach to investigate whether childcare sites that participated in QIS showed greater growth in school readiness of their children than childcare sites that did not participate in QIS over a period of 3 years. After control for site characteristics, the authors found that (a) QIS sites grew significantly faster than non-QIS sites in initial sound fluency and (b) non-QIS sites grew significantly faster than QIS sites in terms of the probability of becoming a low-performing provider. (Contains 4 tables.)