BACKGROUND: Evidence suggests that insulin is under-prescribed in older people. Some reasons for this include physician's concerns about potential side-effects or patients' resistance to insulin. In general, however, little is known about how GPs make decisions related to insulin prescribing in older people. AIM: To explore the process and rationale for prescribing decisions of GPs when treating older patients with type 2 diabetes. DESIGN OF STUDY: Qualitative individual interviews using a grounded theory approach. SETTING: Primary care. METHOD: A thematic analysis was conducted to identify themes that reflected factors that influence the prescribing of insulin. RESULTS: Twenty-one GPs in active practice in Ontario completed interviews. Seven factors influencing the prescribing of insulin for older patients were identified: GPs' beliefs about older people; GPs' beliefs about diabetes and its management; gauging the intensity of therapy required; need for preparation for insulin therapy; presence of support from informal or formal healthcare provider; frustration with management complexity; and GPs' experience with insulin administration. Although GPs indicated that they would prescribe insulin allowing for the above factors, there was a mismatch in intended approach to prescribing and self-reported prescribing. CONCLUSION: GPs' rationale for prescribing (or not prescribing) insulin is mediated by both practitioner-related and patient-related factors. GPs intended and actual prescribing varied depending on their assessment of each patient's situation. In order to improve prescribing for increasing numbers of older people with type 2 diabetes, more education for GPs, specialist support, and use of allied health professionals is needed.