Importance: The past 4 decades have seen substantial changes in allogeneic blood or marrow transplantation (BMT) practice, with the overarching goal of expanding the eligible patient pool while optimizing disease-free survival. Objective: To determine trends in life expectancy and cause-specific late mortality after allogeneic BMT performed over a 40-year period. Design, Setting, and Participants: A retrospective cohort study of 4741 individuals who lived 2 or more years after allogeneic BMT performed between January 1, 1974, and December 31, 2014, was conducted at City of Hope, University of Minnesota, or University of Alabama at Birmingham. The end of follow-up was March 23, 2020. Exposures: Allogeneic BMT performed in 3 eras: 1974-1989, 1990-2004, and 2005-2014. Main Outcomes and Measures: All-cause, recurrence-related, and nonrecurrence-related mortality and projected reduction in life expectancy. Information regarding vital status and cause of death was obtained from the National Death Index Plus and Accurint databases. Results: Of the 4741 individuals included in the study, 2735 (57.7%) were male; median age at BMT was 33 years (range, 0-75 years). The cumulative incidence of recurrence-related mortality plateaued at 10 years, reaching 12.2% (95% CI, 11.0%-13.4%) at 30 years from BMT. In contrast, the incidence of nonrecurrence-related mortality continued to increase and was 22.3% (95% CI, 20.4%-24.3%) at 30 years. Leading causes of nonrecurrence-related mortality included infection (30-year cumulative incidence, 10.7%; standardized mortality ratio [SMR], 52.0), subsequent malignant neoplasms (30-year cumulative incidence, 7.0%; SMR, 4.8), cardiovascular disease (30-year cumulative incidence, 4.6%; SMR, 4.1), and pulmonary disease (30-year cumulative incidence, 2.7%; SMR, 13.9). Compared with the general population, the relative mortality remained higher at 30 or more years after BMT (SMR, 5.4; 95% CI, 4.0-7.1). The cohort experienced a 20.8% reduction in life expectancy (8.7 years of life lost). Compared with 1974-1989 (reference), the adjusted 10-year hazard ratio (HR) of all-cause mortality declined over the 3 eras (1990-2004: HR, 0.67; 95% CI, 0.53-0.85; 2005-2014: HR, 0.52; 95% CI, 0.39-0.69; P <.001 for trend), as did years of life lost (1974-1989: 9.9 years [reference]; 1990-2004: 6.5 years; and 2005-2014: 4.2 years). The reduction in late mortality was most pronounced among individuals who underwent transplantation at ages younger than 18 years (1990-2004: HR, 0.62; 95% CI, 0.40-0.96; 2005-2014: HR, 0.30; 95% CI, 0.16-0.54; reference: 1974-1989; P <.001 for trend) and those who received bone marrow (1990-2004: HR, 0.70; 95% CI, 0.54-0.90; 2005-2014: HR, 0.45; 95% CI, 0.29-0.69; reference: 1974-1989; P <.001 for trend). Conclusions and Relevance: This cohort study noted that late mortality among recipients of allogeneic BMT has decreased over the past 40 years; however, life expectancy was not restored to expected rates compared with the general US population. Furthermore, the reduction in risk of late mortality appeared to be confined to those who underwent transplantation at a younger age or those who received bone marrow. Further efforts to mitigate disease recurrence, infections, subsequent neoplasms, cardiovascular disease, and pulmonary disease may be useful in this population..