OBJECTIVE: The Canadian Diabetes Association recommends that people with diabetes choose foods with low-glycemic index (GI). This study developed a questionnaire measuring Theory of Planned Behaviour (TPB) constructs relative to consuming a low-GI diet by people with diabetes so as to achieve a better understanding of which TPB constructs, demographic characteristics and diabetes-related variables best predict intention to consume a low-GI diet. METHOD: A questionnaire to measure intentions to consume a low-GI diet was developed based on TPB constructs and was administered to 369 adults (30 to 75 years) with type 1 or type 2 diabetes. Responses were analyzed using multiple linear regression. RESULTS: More than 90% of participants (mean age, 56.5±10.8 years; mean body mass index, 30.5±7.2 kg/m(2)) cited reduction and maintenance of healthy blood glucose levels as an advantage of eating low-GI foods. Older age, higher income, female gender, having type 2 diabetes, diabetes treatment (diet only) and understanding of the GI were positively associated with intention to eat a low-GI diet. TPB constructs that significantly predicted intentions to eat a low-GI diet were instrumental attitude (beta = 0.24, p<0.001); subjective norms (beta = 0.13, p=0.007); and perceived behavioural control (beta = 0.55, p<0.001). CONCLUSIONS: This new questionnaire is a valid tool to assess TPB constructs contributing to intentions to eat a low-GI diet by people with diabetes. Future studies that use this questionnaire can shed light on how TPB concepts in clinical practice can help people with diabetes to change their dietary intake.