The study estimates the rate and cost of preventable readmissions within 6 months after a first preventable admission, by age-group, and by payer and race within age-group. The descriptive results are contrasted with several hypotheses. The hospital discharge data are for residents of New York, Pennsylvania, Tennessee, and Wisconsin in 1999, from files of the Healthcare Cost and Utilization Project of the Agency for Healthcare Research and Quality. About 19 percent of persons with an initial preventable admission had at least one preventable readmission rate within 6 months. Hospital cost for preventable readmissions during 6 months was about 730 million US dollars. There were substantial differences in readmission rates by payer group and by race. Some evidence suggests that preventable readmissions may partly reflect complexity of underlying problems. Interventions to reduce cost might focus on identifying high-risk patients before discharge and devising new approaches to follow-up.