Objective To investigate the impact of modifications to contemporary cancer protocols, which minimize exposures to cardiotoxic treatments and preserve long term health, on serious cardiac outcomes among adult survivors of childhood cancer. Design Retrospective cohort study. Setting 27 institutions participating in the Childhood Cancer Survivor Study. Participants 23 462 five year survivors (6193 (26.4%) treated in the 1970s, 9363 (39.9%) treated in the 1980s, and 7906 (33.6%) treated in the 1990s) of leukemia, brain cancer, Hodgkin lymphoma, non-Hodgkin lymphoma, renal tumors, neuroblastoma, soft tissue sarcomas, and bone sarcomas diagnosed prior to age 21 years between 1 January 1970 and 31 December 1999. Median age at diagnosis was 6.1 years (range 0-20.9) and 27.7 years (8.2-58.3) at last follow-up. A comparison group of 5057 siblings of cancer survivors were also included. Main outcome measures Cumulative incidence and 95% confidence intervals of reported heart failure, coronary artery disease, valvular heart disease, pericardial disease, and arrhythmias by treatment decade. Events were graded according to the National Cancer Institute’s Common Terminology Criteria for Adverse Events. Multivariable subdistribution hazard models were used to estimate hazard ratios by decade, and mediation analysis examined risks with and without exposure to cardiotoxic treatments. Results The 20 year cumulative incidence of heart failure (0.69% for those treated in the 1970s, 0.74% for those treated in the 1980s, 0.54% for those treated in the 1990s) and coronary artery disease (0.38%, 0.24%, 0.19%, respectively), decreased in more recent eras (P<0.01), though not for valvular disease (0.06%, 0.06%, 0.05%), pericardial disease (0.04%, 0.02%, 0.03%), or arrhythmias (0.08%, 0.09%, 0.13%). Compared with survivors with a diagnosis in the 1970s, the risk of heart failure, coronary artery disease, and valvular heart disease decreased in the 1980s and 1990s but only significantly for coronary artery disease (hazard ratio 0.65, 95% confidence interval 0.45 to 0.92 and 0.53, 0.36 to 0.77, respectively). The overall risk of coronary artery disease was attenuated by adjustment for cardiac radiation (0.90, 0.78 to 1.05), particularly among survivors of Hodgkin lymphoma (unadjusted for radiation: 0.77, 0.66 to 0.89; adjusted for radiation: 0.87, 0.69 to 1.10). Conclusions Historical reductions in exposure to cardiac radiation have been associated with a reduced risk of coronary artery disease among adult survivors of childhood cancer. Additional follow-up is needed to investigate risk reductions for other cardiac outcomes. Trial registration ClinicalTrials.gov NCT01120353 .