This study describes patterns of long-term weight changes among patients with type 2 diabetes mellitus (DM) and identifies factors associated with weight changes. During the mean follow-up period of 9.1 (SD 3.6; range 1-15) years, 7,820 patients with type 2 DM have been examined to determine weight changes. Their weight at the last clinic visit was compared with that at the initial visit. The mean age of the participants was 51.3 (SD 10.5) years, with a mean DM duration of 6.1 (SD 5.6) years at initial registration. Mean weight change was −1.9% (95% confidence interval, CI −2.1 to −1.7) and varied according to the severity of baseline obesity (-4.1%; 95% CI −4.6 to −3.6) for normal weight, −8.9% (95% CI −13.0 to −4.8) for underweight, −1.7% (95% CI −2.1 to −1.3) for overweight and −0.3% (95% CI −0.03 to +0.73) for obese, and +0.2% (95% CI −1.8 to +2.1) for morbidly obese patients. Using a stepwise multiple regression model, higher body mass index, follow-up, fasting plasma glucose, systolic blood pressure, triglyceride level and treatment with insulin increased the percent weight change, and higher number of follow-up, cholesterol and smoking significantly decreased it. Although this Iranian patients with type 2 DM had negligible weight change over 9.1 years on average, several clinical and lifestyle characteristics were associated with weight change.