Objective: This study assessed osteoporosis knowledge deficits among cancer patients and their spouses/partners. Design: Single-institution survey (modified version of the Osteoporosis Knowledge Assessment Tool). Setting: The Mayo Clinic in Rochester, Minnesota. Participants: Consecutive chemotherapy-treated cancer patients (n = 285) with their spouses/partners (n = 101). Outcome Measures: The main outcome was the percentage of cancer patients who incorrectly conveyed that 1) cancer treatment strengthens bones (or did not know) and/or 2) male cancer patients are not at risk for osteoporosis (or did not know). Analyses: Test scores and 95% confidence intervals (CI) as well as the correlation between patient and spouse/partner scores, are reported. Results: 39% of patients (95% CI, 32% - 48%) thought cancer treatment strengthened bones or did not know, and 39% (95% CI, 32% - 48%) either answered that osteoporosis almost never occurred in men or did not know. The mean correct score on the modified Osteoporosis Knowledge Assessment Tool was 6.7 (95% CI, 6.7, 7.9), and scores from patients correlated with companion scores (r = 0.42; P less than 0.001). Conclusions and Implications: Chemotherapy-treated cancer patients and their companions have knowledge deficits concerning osteoporosis. Educational initiatives to increase awareness may be of value.