The obesity epidemic appears set to worsen the morbidity and mortality from leading causes of death in Australia - ischaemic heart disease, stroke and obesity-related cancers. The aim of this study was to compare hospital separations, deaths and direct health costs for middle-aged adults (45 to 54 years) in 2004/05 with those attaining age 45 to 54 years in 2024/25 who were born into an obesogenic environment. Using data from National Health Surveys, prevalence of obesity in 2004/05 was calculated for those born in 1950/51-59/60 and four scenarios were considered to project rates in 2024/25 for those born in 1970/71-79/80: an age-cohort model; a linear trend model; a steady state where rates increase to equal those of the older birth cohort at the same age; and a best case where rates remain at 2004/05 levels. Population attributable fractions were calculated by gender and disease using relative risks of disease from the literature, and applied to hospital separations, deaths, and direct health system costs data to estimate the proportion of each attributable to obesity. In 2024/25 the projected number of hospitalizations of 45 to 54 year olds due to the diseases of interest could be more than halved, over 200 lives rescued and $51.5 million (in 2004/05 dollars) saved if further gains in obesity in the younger birth cohort are halted. Instead, if the worst case scenario is realized there will be a more than doubling in costs (in 2004/05 dollars) compared with those born in 1950/51-59/60.