© 2017 by American Society of Clinical Oncology. Purpose Cognitive impairment is well-recognized after myeloablative allogeneic hematopoietic cell transplantation (HCT). However, cognitive functioning after reduced-intensity allogeneic or autologous HCT remains unclear. Methods A total of 477 HCT recipients (236 autologous, 128 reduced-intensity allogeneic, 113 myeloablative allogeneic) underwent standardized neuropsychologic testing before HCT and at 6 months and 1, 2, and 3 years after HCT. Ninety-nine frequency-matched healthy controls underwent testing at commensurate time points. Primary outcomes of the study were practice effect-adjusted domainspecific T scores and global deficit scores. Piecewise generalized estimating equation models were used to compare groups and to identify associated variables and post-HCT trends of cognitive impairment. Results Median age was 52 years (range, 18 to 74 years) for HCT recipients and 55 years (range, 19 to 73 years) for controls. Post-HCT scores were comparable between controls and autologous and reduced-intensity HCT recipients. Myeloablative HCT recipients had significantly lower (P > .001) post-HCT scores than controls for executive function, verbal speed, processing speed, auditory memory, and fine motor dexterity. Pre-HCT to 6 months post-HCT scores did not change after reduced-intensity HCT but declined significantly for fine motor dexterity (P > .001) after myeloablative HCT. However, pre-HCT to 3 years post-HCT scores declined significantly (P < .003) in reduced-intensity HCT recipients for executive function, verbal fluency, and working memory. Older age, male sex, and lower education, income, and cognitive reserve were associated with post-HCT cognitive impairment. At 3 years post-HCT, global cognitive impairment was present in 18.7% of autologous and 35.7% of allogeneic HCT recipients. Conclusion Myeloablative allogeneic HCT recipients showed significant cognitive decline compared with healthy controls. Reduced-intensity allogeneic HCT recipients showed evidence of delayed decline. Cognitive functioning in autologous HCT recipients generally was spared. The study identified vulnerable subpopulations that could benefit from targeted interventions.