To evaluate the effect of fluctuations in waist circumference (WC), weight, and body mass index (BMI) on the incidence of diabetes in older adults. A prospective cohort of 61,587 older adults (age, 60-96 years) who did not have diabetes at study initiation was examined. Data on weight, BMI, and WC were collected, and participants were followed up until 31 December 2018. The main end point was new-onset diabetes. A Cox regression model was used to estimate the risk of diabetes (hazard ratios HRs and confidence intervals CI) in these participants. During a mean follow-up of 3.6 years, being overweight (HR 95% CI 1.87 1.62-2.17), obesity (1.41 1.26-1.59), abdominal obesity (1.42 1.28-1.58), and obesity plus abdominal obesity at baseline (1.93 1.66-2.25) increased the risk of diabetes onset. Compared with older adults who "maintained normal WC", those who "remained abdominally obese" (HR = 1.66), "became abdominally obese" (HR = 1.58), or "achieved normal WC" (HR = 1.36) were at a higher risk of diabetes onset, as well as those with an increase in WC > 3 cm or > 5% compared with the baseline level. Weight gain or loss > 6 kg or weight gain > 5%, increase or decrease in BMI > 2 kg/m.sup.2, or an increase in BMI > 10% were associated with a higher diabetes risk. The diabetes risk was reduced by 19% in overweight older adults who exercised daily. For older adults, WC, BMI, and healthy weight maintenance reduce the diabetes risk. The findings may provide evidence for developing guidelines of proper weight and WC control for older adults.