Cancer presents unique challenges to the clinician who hopes to achieve patient adherence to medical recommendations. Preventive and treatment regimens, particularly those that involve lifestyle change, are often extremely difficult to carry out (eg, smoking cessation; following a low fat diet). Although research suggests that there is no clearcut relationship between a patient's age and his or her degree of adherence to medical treatment, older patients may have more difficulties than younger persons in understanding precisely the medical recommendations made to them and tend to be more passive recipients of care. The authors describe six factors that significantly affect adherence and clinical changes that are likely to be effective in enhancing the adherence behavior of older patients with cancer. Measurement of patient adherence in the Medical Outcomes Study is described. This paper also examines the difficulties and pressures faced by medical professionals in enhancing the adherence of their patients.