African-American (AA) cancer patients have long-experienced worse outcomes compared to non-Hispanic whites (NHW). No studies to date have evaluated the prognostic impact of sickle cell trait (SCT) and other inherited haemoglobinopathies, of which several are disproportionately high in the AA population. In a cohort analysis of treated patients diagnosed with breast or prostate cancer in the linked SEER-Medicare database, the relative risk (RR) for ≥1 serious adverse events (AEs), defined as hospitalisations or emergency department visits, was estimated for 371 AA patients with a haemoglobinopathy (AA+) compared to patients without haemoglobinopathies (17,303 AA-; 144,863 NHW-). AA+ patients had significantly increased risk for ≥1 AEs compared to AA- (RR = 1.19; 95% CI 1.11-1.27) and NHW- (RR = 1.23; 95% CI 1.15-1.31) patients. The magnitude of effect was similar by cancer type, and in analyses of AA+ with SCT only. Our findings suggest a novel hypothesis for disparities in cancer outcomes.