Background There is growing interest in how frequently patients undergoing coronary artery bypass graft (CABG) surgery require hospital readmission within 30 days of discharge. Readmissions, however, may not capture all hospital-based, acute care needs after discharge. The purpose of this study is to describe the frequency of and diagnoses associated with emergency department (ED) visits and hospital readmissions within 30 days of discharge after CABG surgery and to compare outcomes across hospitals. Methods Using the California State Inpatient and Emergency Department Databases, we identified all adults who underwent isolated CABG surgery between January 2005 and June 2009. We then calculated hospitals' 30-day, risk-standardized readmission and ED visit rates using hierarchic generalized linear models. The correlation between hospital readmission and ED visit rates was estimated, weighting for hospital volume. Results We identified 63,911 adults who underwent isolated CABG surgery at 114 hospitals. Hospital 30-day, risk-standardized ED visit without readmission rates (median ED visit rate = 11.9%, 25th to 75th percentile, 10.5% to 13.7%) nearly equaled the hospital 30-day risk-standardized readmission rates (median readmission rate = 15.0%, 25th to 75th percentile, 13.5% to 16.5%). Both outcomes varied widely among hospitals. A composite of these outcomes, the median 30-day risk-standardized hospital-based, acute care rate was 23.9% (25th to 75th percentile, 22.2% to 25.5%). Postoperative infections, congestive heart failure, and chest discomfort were among the most common reasons for both readmissions and ED visits. Hospitals' 30-day risk-standardized ED visit and readmission rates were not significantly correlated (weighted correlation coefficient = −0.07, p = 0.44). Conclusions Patients discharged after CABG surgery frequently experienced ED visits and hospital readmissions within 30 days, often for similar diagnoses. Monitoring both hospital readmissions and ED visits after CABG surgery is important to our understanding of hospital-based, acute care needs after discharge.