Objectives: Potentially inappropriate medication (PIM) use has been associated with an increase in adverse drug events, hospitalization and mortality. This study investigated the prevalence and factors associated with PIM use in patients presenting to a medical oncology outpatient clinic. Materials and Methods: Consecutive patients (n= 385) aged = 70. years referred to a medical oncology outpatient clinic between January 2009 and July 2010 completed a structured data collection instrument. The instrument assessed medication use, diagnoses, self-reported falls in the previous six months, pain (10-point visual analog scale [VAS]) and distress (10-point VAS). Frailty was defined using exhaustion, weight loss, Karnofsky Performance Scale, instrumental activities of daily living and physical function. PIM use was defined by the Beers Criteria. Logistic regression was used to compute odds ratios (ORs) and 95 confidence intervals (CIs) for factors associated with PIM use. Results: In total, 26.5 (n= 102) of the sample used = 1 PIM. The five most prevalent classes of PIMs were benzodiazepines (n= 34, 8.8 ), tricyclic antidepressants (n= 16, 4.2 ), alpha-adrenoreceptor antagonists (prazosin) (n= 15, 3.9 ), propulsives (metoclopramide) (n= 15, 3.9 ) and non-steroidal anti-inflammatory drugs (n= 14, 3.6 ). In multivariate analyses, PIM use was associated with age 75-79. years (OR 1.83; 95 CI 1.02-3.26) compared to age 70-74. years, using = 5 medications (OR 4.10; 95 CI 2.26-7.44) compared to <5 medications and being frail (OR 3.05; 95 CI 1.18-7.87) compared to being robust. Conclusion: More than one quarter of older people with cancer used one or more PIMs, and this was associated with being frail compared to being robust.