Background: ASPirin in Reducing Events in the Elderly, a randomized, double-blind, placebo-controlled trial of daily low-dose aspirin (100 mg) in older adults, showed an increase in all-cause mortality, primarily due to cancer. In contrast, prior randomized controlled trials, mainly involving younger individuals, demonstrated a delayed cancer benefit with aspirin. We now report a detailed analysis of cancer incidence and mortality. Methods: 19 114 Australian and US community-dwelling participants aged 70 years and older (US minorities 65 years and older) without cardiovascular disease, dementia, or physical disability were randomly assigned and followed for a median of 4.7 years. Fatal and nonfatal cancer events, a prespecified secondary endpoint, were adjudicated based on clinical records. Results: 981 cancer events occurred in the aspirin and 952 in the placebo groups. There was no statistically significant difference between groups for all incident cancers (hazard ratio [HR] = 1.04, 95% confidence interval [CI] = 0.95 to 1.14), hematological cancer (HR = 0.98, 95% CI = 0.73 to 1.30), or all solid cancers (HR = 1.05, 95% CI = 0.95 to 1.15), including by specific tumor type. However, aspirin was associated with an increased risk of incident cancer that had metastasized (HR = 1.19, 95% CI = 1.00 to 1.43) or was stage 4 at diagnosis (HR = 1.22, 95% CI = 1.02 to 1.45), and with higher risk of death for cancers that presented at stages 3 (HR = 2.11, 95% CI = 1.03 to 4.33) or 4 (HR = 1.31, 95% CI = 1.04 to 1.64). Conclusions: In older adults, aspirin treatment had an adverse effect on later stages of cancer evolution. These findings suggest that in older persons, aspirin may accelerate the progression of cancer and, thus, suggest caution with its use in this age group.