IMPORTANCE: Advanced age is associated with lower use of drug treatment to prevent fractures, but concerns about comorbidities and prognosis increase the complexity of managing osteoporosis in this age group. OBJECTIVE: To determine the association of disease definition, number of comorbidities, and prognosis with 5-year hip fracture probabilities among women who are 80 years and older. DESIGN, SETTING, AND PARTICIPANTS: This prospective cohort study (4 US sites) included 1528 community-dwelling women identified as potential candidates for initiation of osteoporosis drug treatment. MAIN OUTCOMES AND MEASURES: Women were contacted every 4 months to ascertain vital status and hip fracture. Five-year hip fracture probability was calculated accounting for competing mortality risk. Participants were classified into 2 distinct groups based on disease definition criteria proposed by the National Bone Health Alliance: with osteoporosis (n = 761) and without osteoporosis but at high fracture risk (n = 767). Comorbid conditions were assessed by self-report. Prognosis was estimated using a mortality prediction index. All analysis was performed between March 2018 and January 2019. RESULTS: The study had 1528 participants, all of whom were women, with a mean (SD) age of 84.1 (3.4) years. During follow-up, 125 (8.0%) women experienced a hip fracture and 287 (18.8%) died before experiencing this event. Five-year mortality probability was 24.9% (95% CI, 21.8-28.1) among women with osteoporosis and 19.4% (95% CI, 16.6-22.3) among women without osteoporosis but at high fracture risk. In both groups, mortality probability similarly increased with more comorbidities and poorer prognosis. In contrast, 5-year hip fracture probability was 13.0% (95% CI, 10.7-15.5) among women with osteoporosis and 4.0% (95% CI, 2.8-5.6) among women without osteoporosis but at high fracture risk. The difference was most pronounced among women with more comorbidities or worse prognosis. For example, among women with 3 or more comorbid conditions, hip fracture probability was 18.1% (95% CI, 12.3-24.9) among women with osteoporosis vs 2.5% (95% CI, 1.3-4.2) among women without osteoporosis but at high fracture risk. CONCLUSIONS AND RELEVANCE: Women 80 years and older who have osteoporosis, including those with more comorbidities or poorer prognosis, have a high 5-year probability of hip fracture despite accounting for competing mortality risk. In contrast, among women without osteoporosis but at high fracture risk, competing mortality risk far outweighs hip fracture probability, especially among those with more comorbidities or worse prognosis.