Several country-specific and global projections of the future obesity prevalence have been conducted. However, these projections are obtained by extrapolating past prevalence of obesity or distributions of body weight. More accurate would be to base estimates on the most recent measures of weight change. Using measures of overweight and obesity incidence from a national, longitudinal study, we estimated the future obesity prevalence in Australian adults. Participants were adults aged ≥25 years in 2000 participating in the Australian Diabetes, Obesity, and Lifestyle (AusDiab) study (baseline 2000, follow-up 2005). In this population, approximately one-fifth of those with normal weight or overweight progressed to a higher weight category within 5 years. Between 2000 and 2025, the adult prevalence of normal weight was estimated to decrease from 40.6 to 28.1% and the prevalence of obesity to increase from 20.5 to 33.9%. By the time, those people aged 25–29 in 2000 reach 60–64 years, 22.1% will be normal weight, and 42.4% will be obese. On average, normal-weight females aged 25–29 years in 2000 will live another 56.2 years: 26.6 years with normal weight, 15.6 years with overweight, and 14.0 years with obesity. Normal-weight males aged 25–29 years in 2000 will live another 51.5 years: 21.6 years with normal weight, 21.1 years with overweight, and 8.8 years with obesity. If the rates of weight gain observed in the first 5 years of this decade are maintained, our findings suggest that normal-weight adults will constitute less than a third of the population by 2025, and the obesity prevalence will have increased by 65%.