e21545 Background: A nocebo is an inert substance that causes adverse events. Although previous studies have examined the favorably positive (placebo) effects of an inert substance, few studies have examined negative (nocebo) effects, particularly in older patients who sometimes experience frequent and severe adverse events from cancer therapy. Methods: This study focused on placebo/nocebo-exposed patients who participated in two double-blind, placebo-controlled, cancer therapeutic studies, namely, North Central Cancer Treatment Group (NCCTG) 97-24-51 and American College of Surgeons Oncology Group (ACOSOG) Z9001, with the goal of reporting comparative, age-based adverse event rates. Results: Among the 446 patients who received placebo/nocebo exclusively, 161 were 65 years of age or older at the time of respective trial entry; 5234 adverse events occurred. Unadjusted adverse event rates did not differ significantly between patients 65 years of age or older and those younger: rate ratio (99% confidence intervals (CI): 1.01 (0.47, 2.02) with similar findings for grade 2 or worse adverse events and for all symptom-driven adverse events (for example, pain, loss of appetite, anxiety). Adjustment for sex, ethnicity, baseline performance score, and trial resulted in no significant age-based rate differences in adverse event rates. Similar findings were observed with an age threshold of 70. Conclusions: A nocebo effect appears to occur irrespective of age. This observation suggests that adverse events should be taken no less seriously in older than in younger cancer patients and that education to manage patients’ expectations from cancer therapy might improve tolerability. [Table: see text]