OBJECTIVES: The objective was to understand how type-2 diabetes mellitus (T2DM) patients and their physicians perceive weight in relation to the objective measure of body mass index (BMI). METHODS: Analysis was performed using data from the Adelphi Diabetes Disease Specific Programme (DSP©), a large cross-sectional study of clinical and treatment practice. Physicians (diabetologists and primary care practitioners) provided clinical assessment and information on T2DM patients. All physicians and consenting patients were required, separately, to categorise patient weight as: normal, overweight, severely overweight/obese, severely obese. Physicians also provided height and weight for patients but were not required to calculate BMI. Physician and patient estimates were subsequently compared with calculated BMI for each patient. RESULTS: A total of 132 (US) and 511 (5 EU countries) physicians participated. Physicians categorised the weight of 928 (US) and 3969 (EU) T2DM patients for whom BMI was calculated. Equivalent matched numbers of patients categorising their weight was 626 (US) and 2013 (EU). Patient mean age was 62 years; duration of T2DM 6 years; 51% were male; 55% in US and 33% in EU were obese (BMI>/=30). Across all weights, some patients' weight category was underestimated by their physicians (US 38% underestimated vs. 54% correct; EU 21% vs. 69%). T2DM patients were more likely to underestimate their weight (US 60% underestimated vs. 37% correct, EU 43% vs. 51%). Discrepancies in weight estimation were greatest for countries where overall mean BMI was >30 (US, UK) and least where mean BMI was lowest (Italy). CONCLUSIONS: Weight control is an important aspect of management of T2DM. This requires accurate assessment and agreement of patient weight and BMI levels by treating physicians and their patients. Accurate risk stratification based on weight and BMI may help improve effective communications and disease management decisions between T2DM patients and their treating physicians.