OBJECTIVE: To determine whether there are differences between family physicians' beliefs and treatment intentions regarding older patients with mental disorders and younger patients with similar disorders. Such differences might contribute to older adults' lower rates of mental health service use. DESIGN: Mailed survey. SETTING: Primary care practices in and around Kingston, Ont. PARTICIPANTS: Questionnaires were mailed to 294 general practitioners listed in the 42nd Annual Canadian Medical Directory. Of the 285 eligible physicians, 115 (40%) completed and returned questionnaires. MAIN OUTCOME MEASURES: Physicians' ratings of preparedness to identify and treat, likelihood of treating, likelihood of using each of five different treatment methods, likelihood of referral, preferences for six referral options, and treatment effectiveness with respect to hypothetical older and younger patients with panic disorder or dysthymia. RESULTS: Physicians reported being less prepared to identify and treat older patients than younger patients. In addition, physicians reported being significantly less likely to treat and to refer older patients than younger patients. Finally, physicians reported that both psychotherapy alone, and in combination with pharmacotherapy, were less effective for older patients than for younger patients. CONCLUSIONS: In addition to other possible reasons for older adults' low rates of mental health service use, this study suggests that family physicians' beliefs and treatment intentions could be contributing factors. Changes in medical education aimed at replacing inaccurate beliefs with accurate information regarding older patients might be one way to increase rates of use in this underserved age group, because family physicians play a key role in the mental health care of older adults.